ECMAScript
2020,第11版,引入了字符串的matchAll方法,用于生成全局正则表达式匹配对象的迭代器;import()语法,用于动态异步导入模块;BigInt,一种新的数字原始类型,用于处理任意精度的整数;Promise.allSettled,一种新的Promise组合器,不会短路;globalThis,一种通用的全局this值访问方式;专用的export * as ns from 'module'语法,用于模块内;增加了for-in枚举顺序的标准化;import.meta,一个由宿主填充的对象,包含关于模块的上下文信息;以及为处理“nullish”值(undefined或null)引入的两个新语法特性:nullish合并,一个值选择操作符;以及可选链,一个属性访问和函数调用操作符,当要访问/调用的值为nullish时会短路。
ECMAScript 2022,第13版,引入了顶层await,允许在模块顶层使用关键字;新的类元素:公共和私有实例字段、公共和私有静态字段、私有实例方法和访问器、以及私有静态方法和访问器;类中的静态块,用于每类评估初始化;#x in obj语法,用于测试对象上私有字段的存在;通过/d标志的正则表达式匹配索引,提供匹配子字符串的起始和结束索引;Error对象上的cause属性,用于记录错误中的因果链;用于字符串、数组和类型化数组的at方法,允许相对索引;以及Object.hasOwn,Object.prototype.hasOwnProperty的便捷替代。
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies.
IEEE
754-2019, IEEE Standard for Floating-Point Arithmetic.
ISO/IEC 10646, Information Technology — Universal Multiple-Octet Coded Character Set (UCS) plus
Amendment 1:2005, Amendment 2:2006, Amendment 3:2008, Amendment 4:2008, and additional amendments and
corrigenda, or successor.
This section contains a non-normative overview of the ECMAScript language.
ECMAScript is an object-oriented programming language for performing computations and manipulating
computational objects within a host environment. ECMAScript as defined
here is not intended to be computationally self-sufficient; indeed, there are no provisions in this
specification for input of external data or output of computed results. Instead, it is expected that the
computational environment of an ECMAScript program will provide not only the objects and other facilities
described in this specification but also certain environment-specific objects, whose description and behaviour
are beyond the scope of this specification except to indicate that they may provide certain properties that can
be accessed and certain functions that can be called from an ECMAScript program.
ECMAScript was originally designed to be used as a scripting language, but has become widely used as a
general-purpose programming language. A scripting language is a programming language that is used to
manipulate, customize, and automate the facilities of an existing system. In such systems, useful functionality
is already available through a user interface, and the scripting language is a mechanism for exposing that
functionality to program control. In this way, the existing system is said to provide a host
environment of objects and facilities, which completes the capabilities of the scripting
language. A scripting language is intended for use by both professional and non-professional programmers.
ECMAScript was originally designed to be a Web scripting language, providing a mechanism to enliven
Web pages in browsers and to perform server computation as part of a Web-based client-server architecture.
ECMAScript is now used to provide core scripting capabilities for a variety of host
environments. Therefore the core language is specified in this document apart from any
particular host environment.
ECMAScript usage has moved beyond simple scripting and it is now used for the full spectrum of programming
tasks in many different environments and scales. As the usage of ECMAScript has expanded, so have the features
and facilities it provides. ECMAScript is now a fully featured general-purpose programming language.
4.1 Web Scripting
A web browser provides an ECMAScript host environment for client-side
computation including, for instance, objects that represent windows, menus, pop-ups, dialog boxes, text areas,
anchors, frames, history, cookies, and input/output. Further, the host environment
provides a means to attach scripting code to events such as change of focus, page and image loading,
unloading, error and abort, selection, form submission, and mouse actions. Scripting code appears within the
HTML and the displayed page is a combination of user interface elements and fixed and computed text and
images. The scripting code is reactive to user interaction, and there is no need for a main program.
A web server provides a different host environment for server-side
computation including objects representing requests, clients, and files; and mechanisms to lock and share
data. By using browser-side and server-side scripting together, it is possible to distribute computation
between the client and server while providing a customized user interface for a Web-based application.
Each Web browser and server that supports ECMAScript supplies its own host environment,
completing the ECMAScript execution environment.
4.2 Hosts and Implementations
To aid integrating ECMAScript into host environments, this specification
defers the definition of certain facilities (e.g., abstract
operations), either in whole or in part, to a source outside of this specification.
Editorially, this specification distinguishes the following kinds of deferrals.
An implementation is an external source that further defines facilities
enumerated in Annex D or those that are marked as
implementation-defined or
implementation-approximated.
In informal use, an implementation refers to a concrete artefact, such as a particular web browser.
An implementation-defined facility is one that defers
its definition to an external source without further qualification. This specification does not make any
recommendations for particular behaviours, and conforming implementations are free to choose any behaviour
within the constraints put forth by this specification.
An implementation-approximated facility is one that
defers its definition to an external source while recommending an ideal behaviour. While conforming
implementations are free to choose any behaviour within the constraints put forth by this specification, they
are encouraged to strive to approximate the ideal. Some mathematical operations, such as Math.exp, are implementation-approximated.
A host is an external source that further defines
facilities listed in Annex D but does not further define
other implementation-defined or
implementation-approximated
facilities. In informal use, a host refers to the set of all implementations,
such as the set of all web browsers, that interface with this specification in the same way via Annex
D. A host is often an external
specification, such as WHATWG HTML (https://html.spec.whatwg.org/). In other words, facilities that are
host-defined are often further defined in
external specifications.
A host hook is an abstract operation that is
defined in whole or in part by an external source. All host hooks must be listed in Annex D. A host hook
must conform to at least the following requirements:
A host-defined facility is one that defers its definition to an
external source without further qualification and is listed in Annex D. Implementations that are
not hosts
may also provide definitions for host-defined facilities.
A host environment is a
particular choice of definition for all host-defined facilities. A host
environment typically includes objects or functions which allow obtaining input and
providing output as host-defined properties of the global
object.
This specification follows the editorial convention of always using the most specific term. For example, if a
facility is host-defined, it should not be referred
to as implementation-defined.
Both hosts
and implementations may interface with this specification via the language types, specification types,
abstract
operations, grammar productions, intrinsic objects, and intrinsic symbols defined herein.
4.3 ECMAScript Overview
The following is an informal overview of ECMAScript—not all parts of the language are described. This
overview is not part of the standard proper.
ECMAScript is object-based: basic language and host facilities are provided by objects, and an
ECMAScript program is a cluster of communicating objects. In ECMAScript, an object is a collection of
zero or more properties each with attributes that determine how each property can be
used—for example, when the Writable attribute for a property is set to false, any attempt
by executed ECMAScript code to assign a different value to the property fails. Properties are containers that
hold other objects, primitive values, or functions. A primitive value is a member of one of
the following built-in types: Undefined, Null, Boolean, Number, BigInt,
String, and Symbol; an object is a member of the built-in type Object; and a function is
a callable object. A function that is associated with an object via a property is called a method.
ECMAScript defines a collection of built-in objects that round out the definition of ECMAScript
entities. These built-in objects include the global object; objects that are
fundamental to the runtime semantics of the
language including Object, Function, Boolean, Symbol, and
various Error objects; objects that represent and manipulate numeric values including
Math, Number, and Date; the text processing objects String
and RegExp; objects that are indexed collections of values including Array and nine
different kinds of Typed Arrays whose elements all have a specific numeric data representation; keyed
collections including Map and Set objects; objects supporting structured data
including the JSON object, ArrayBuffer, SharedArrayBuffer, and
DataView; objects supporting control abstractions including generator functions and
Promise objects; and reflection objects including Proxy and Reflect.
ECMAScript also defines a set of built-in operators. ECMAScript operators include various unary
operations, multiplicative operators, additive operators, bitwise shift operators, relational operators,
equality operators, binary bitwise operators, binary logical operators, assignment operators, and the comma
operator.
Large ECMAScript programs are supported by modules which allow a program to be divided into multiple
sequences of statements and declarations. Each module explicitly identifies declarations it uses that need to
be provided by other modules and which of its declarations are available for use by other modules.
ECMAScript syntax intentionally resembles Java syntax. ECMAScript syntax is relaxed to enable it to serve as
an easy-to-use scripting language. For example, a variable is not required to have its type declared nor are
types associated with properties, and defined functions are not required to have their declarations appear
textually before calls to them.
4.3.1 Objects
Even though ECMAScript includes syntax for class definitions, ECMAScript objects are not fundamentally
class-based such as those in C++, Smalltalk, or Java. Instead objects may be created in various ways
including via a literal notation or via constructors which create objects
and then execute code that initializes all or part of them by assigning initial values to their properties.
Each constructor is a function that has a
property named "prototype" that is used to implement prototype-based inheritance
and shared properties. Objects are created by using constructors in new expressions;
for example, new Date(2009, 11) creates a new Date object. Invoking a constructor without using new has
consequences that depend on the constructor. For example,
Date() produces a string representation of the current date and time rather than an object.
Every object created by a constructor has an implicit reference
(called the object's prototype) to the value of its constructor's
"prototype" property. Furthermore, a prototype may have a non-null
implicit reference to its prototype, and so on; this is called the prototype chain. When a
reference is made to a property in an object, that reference is to the property of that name in the first
object in the prototype chain that contains a property of that name. In other words, first the object
mentioned directly is examined for such a property; if that object contains the named property, that is the
property to which the reference refers; if that object does not contain the named property, the prototype
for that object is examined next; and so on.
Figure 1: Object/Prototype Relationships
In a class-based object-oriented language, in general, state is carried by instances, methods are carried
by classes, and inheritance is only of structure and behaviour. In ECMAScript, the state and methods are
carried by objects, while structure, behaviour, and state are all inherited.
All objects that do not directly contain a particular property that their prototype contains share that
property and its value. Figure 1 illustrates this:
CF is a constructor (and also an object). Five
objects have been created by using new expressions: cf1,
cf2, cf3, cf4, and cf5. Each of
these objects contains properties named "q1" and "q2". The dashed
lines represent the implicit prototype relationship; so, for example, cf3's prototype is
CFp. The constructor, CF, has two
properties itself, named "P1" and "P2", which are not visible to
CFp, cf1, cf2, cf3,
cf4, or cf5. The property named "CFP1" in
CFp is shared by cf1, cf2, cf3,
cf4, and cf5 (but not by CF), as are any properties found in
CFp's implicit prototype chain that are not named "q1",
"q2", or "CFP1". Notice that there is no implicit prototype link
between CF and CFp.
Unlike most class-based object languages, properties can be added to objects dynamically by assigning
values to them. That is, constructors are not required to name or
assign values to all or any of the constructed object's properties. In the above diagram, one could add a
new shared property for cf1, cf2, cf3,
cf4, and cf5 by assigning a new value to the property in
CFp.
Although ECMAScript objects are not inherently class-based, it is often convenient to define class-like
abstractions based upon a common pattern of constructor functions, prototype
objects, and methods. The ECMAScript built-in objects themselves follow such a class-like pattern. Beginning
with ECMAScript 2015, the ECMAScript language includes syntactic class definitions that permit programmers
to concisely define objects that conform to the same class-like abstraction pattern used by the built-in
objects.
4.3.2 The Strict Variant of ECMAScript
The ECMAScript Language recognizes the possibility that some users of the language may wish to restrict
their usage of some features available in the language. They might do so in the interests of security, to
avoid what they consider to be error-prone features, to get enhanced error checking, or for other reasons of
their choosing. In support of this possibility, ECMAScript defines a strict variant of the language. The
strict variant of the language excludes some specific syntactic and semantic features of the regular
ECMAScript language and modifies the detailed semantics of some features. The strict variant also specifies
additional error conditions that must be reported by throwing error exceptions in situations that are not
specified as errors by the non-strict form of the language.
The strict variant of ECMAScript is commonly referred to as the strict mode of the language.
Strict mode selection and use of the strict mode syntax and semantics of ECMAScript is explicitly made at
the level of individual ECMAScript source text units as
described in 11.2.2. Because strict mode is
selected at the level of a syntactic source text unit, strict mode only imposes restrictions that have local
effect within such a source text unit. Strict mode does not restrict or modify any aspect of the ECMAScript
semantics that must operate consistently across multiple source text units. A complete ECMAScript program
may be composed of both strict mode and non-strict mode ECMAScript source text units. In
this case, strict mode only applies when actually executing code that is defined within a strict mode source
text unit.
In order to conform to this specification, an ECMAScript implementation must implement both the full
unrestricted ECMAScript language and the strict variant of the ECMAScript language as defined by this
specification. In addition, an implementation must support the combination of unrestricted and strict mode
source text units into a single composite program.
4.4 Terms and Definitions
For the purposes of this document, the following terms and definitions apply.
4.4.1 implementation-approximated
an implementation-approximated
facility is defined in whole or in part by an external source but has a recommended, ideal behaviour in this
specification
4.4.2 implementation-defined
an implementation-defined
facility is defined in whole or in part by an external source to this specification
The value of a constructor's
"prototype" property is a prototype object that is used to implement inheritance and
shared properties.
4.4.8 prototype
object that provides shared properties for other objects
Note
When a constructor creates an object, that
object implicitly references the constructor's
"prototype" property for the purpose of resolving property references. The constructor's
"prototype" property can be referenced by the program expression
constructor.prototype, and properties added to an object's prototype are shared,
through inheritance, by all objects sharing the prototype. Alternatively, a new object may be created
with an explicitly specified prototype by using the Object.create built-in function.
4.4.9 ordinary object
object that has the default behaviour for the essential internal methods that must be supported by all
objects
4.4.10 exotic object
object that does not have the default behaviour for one or more of the essential internal methods
There are only two Boolean values, true and false.
4.4.18 Boolean type
type consisting of the primitive values true and false
4.4.19 Boolean object
member of the Object type that is an instance of
the standard built-in Boolean constructor
Note
A Boolean object is created by using the Boolean constructor in a new
expression, supplying a Boolean value as an argument. The resulting object has an internal slot whose
value is the Boolean value. A Boolean object can be coerced to a Boolean value.
4.4.20 String value
primitive value that is a finite ordered sequence of zero or more
16-bit unsigned integer values
Note
A String value is a member of the String
type. Each integer value in the sequence usually
represents a single 16-bit unit of UTF-16 text. However, ECMAScript does not place any restrictions or
requirements on the values except that they must be 16-bit unsigned integers.
4.4.21 String type
set of all possible String values
4.4.22 String object
member of the Object type that is an instance of
the standard built-in String constructor
Note
A String object is created by using the String constructor in a new
expression, supplying a String value as an argument. The resulting object has an internal slot whose
value is the String value. A String object can be coerced to a String value by calling the String
constructor as a function (22.1.1.1).
4.4.23 Number value
primitive value corresponding to a double-precision 64-bit binary format IEEE 754-2019 value
Note
A Number value is a member of the Number
type and is a direct representation of a number.
4.4.24 Number type
set of all possible Number values including the special “Not-a-Number” (NaN) value, positive infinity, and
negative infinity
4.4.25 Number object
member of the Object type that is an instance of
the standard built-in Number constructor
Note
A Number object is created by using the Number constructor in a new
expression, supplying a Number value as an argument. The resulting object has an internal slot whose
value is the Number value. A Number object can be coerced to a Number value by calling the Number
constructor as a function (21.1.1.1).
4.4.26 Infinity
Number value that is the positive infinite Number value
4.4.27 NaN
Number value that is an IEEE 754-2019 “Not-a-Number” value
4.4.28 BigInt value
primitive value corresponding to an arbitrary-precision integer value
4.4.29 BigInt type
set of all possible BigInt values
4.4.30 BigInt object
member of the Object type that is an instance of
the standard built-in BigInt constructor
4.4.31 Symbol value
primitive value that represents a unique, non-String Object property key
4.4.32 Symbol type
set of all possible Symbol values
4.4.33 Symbol object
member of the Object type that is an instance of
the standard built-in Symbol constructor
4.4.34 function
member of the Object type that may be invoked as a
subroutine
Note
In addition to its properties, a function contains executable code and state that determine how it
behaves when invoked. A function's code may or may not be written in ECMAScript.
4.4.35 built-in function
built-in object that is a function
Note
Examples of built-in functions include parseInt and Math.exp. A host or
implementation may provide additional built-in functions that are not described in this specification.
Examples of built-in constructors include
Object and Function. A host or implementation may provide
additional built-in constructors that are not described
in this specification.
4.4.37 property
part of an object that associates a key (either a String value or a Symbol value) and a value
Note
Depending upon the form of the property the value may be represented either directly as a data value (a
primitive value, an object, or a function object) or indirectly
by a pair of accessor functions.
4.4.38 method
function that is the value of a property
Note
When a function is called as a method of an object, the object is passed to the function as its
this value.
4.4.39 built-in method
method that is a built-in function
Note
Standard built-in methods are defined in this specification. A host or implementation may provide
additional built-in methods that are not described in this specification.
4.4.40 attribute
internal value that defines some characteristic of a property
4.4.41 own property
property that is directly contained by its object
4.4.42 inherited property
property of an object that is not an own property but is a property (either own or inherited) of the
object's prototype
4.5 Organization of This Specification
The remainder of this specification is organized as follows:
Clause 5 defines the notational
conventions used throughout the specification.
Clauses 6 through
10 define
the execution environment within which ECMAScript programs operate.
Clauses 11 through
17 define
the actual ECMAScript programming language including its syntactic encoding and the execution semantics of all
language features.
Clauses 18 through
28 define the ECMAScript standard
library. They include the definitions of all of the standard objects that are available for use by ECMAScript
programs as they execute.
Clause 29 describes the memory consistency
model of accesses on SharedArrayBuffer-backed memory and methods of the Atomics object.
5 Notational Conventions
5.1 Syntactic and Lexical Grammars
5.1.1 Context-Free Grammars
A context-free grammar consists of a number of productions. Each production has an
abstract symbol called a nonterminal as its left-hand side, and a sequence of zero or more
nonterminal and terminal symbols as its right-hand side. For each grammar, the terminal
symbols are drawn from a specified alphabet.
A chain production is a production that has exactly
one nonterminal symbol on its right-hand side along with zero or more terminal symbols.
Starting from a sentence consisting of a single distinguished nonterminal, called the goal symbol, a given context-free grammar specifies a
language, namely, the (perhaps infinite) set of possible sequences of terminal symbols that can
result from repeatedly replacing any nonterminal in the sequence with a right-hand side of a production for
which the nonterminal is the left-hand side.
Input elements other than white space and comments form the terminal symbols for the syntactic grammar for
ECMAScript and are called ECMAScript tokens. These tokens are the reserved words,
identifiers, literals, and punctuators of the ECMAScript language. Moreover, line terminators, although not
considered to be tokens, also become part of the stream of input elements and guide the process of automatic
semicolon insertion (12.10). Simple
white space and single-line comments are discarded and do not appear in the stream of input elements for the
syntactic grammar. A MultiLineComment (that is, a
comment of the form /*…*/ regardless of whether it spans more than one line) is
likewise simply discarded if it contains no line terminator; but if a MultiLineComment contains one or
more line terminators, then it is replaced by a single line terminator, which becomes part of the stream of
input elements for the syntactic grammar.
A RegExp grammar for ECMAScript is given in 22.2.1. This grammar also has as its
terminal symbols the code points as defined by SourceCharacter. It defines a set
of productions, starting from the goal symbolPattern, that describe
how sequences of code points are translated into regular expression patterns.
Productions of the lexical and RegExp grammars are distinguished by having two colons “::” as
separating punctuation. The lexical and RegExp grammars share some productions.
Productions of the numeric string grammar are distinguished by having three colons “:::” as
punctuation, and are never used for parsing source text.
5.1.4 The Syntactic Grammar
The syntactic grammar for ECMAScript is given in clauses 13 through
16. This
grammar has ECMAScript tokens defined by the lexical grammar as its terminal symbols (5.1.2). It defines a
set of productions, starting from two alternative goal
symbolsScript and Module, that describe how sequences of
tokens form syntactically correct independent components of ECMAScript programs.
When a stream of code points is to be parsed as an ECMAScript Script or Module, it is first converted to a stream
of input elements by repeated application of the lexical grammar; this stream of input elements is then
parsed by a single application of the syntactic grammar. The input stream is syntactically in error if the
tokens in the stream of input elements cannot be parsed as a single instance of the goal nonterminal
(Script or Module), with no tokens
left over.
When a parse is successful, it constructs a parse tree, a rooted tree structure in which each node
is a Parse Node. Each Parse Node is an instance of
a symbol in the grammar; it represents a span of the source text that can be derived from that symbol. The
root node of the parse tree, representing the whole of the source text, is an instance of the parse's
goal symbol. When a Parse
Node is an instance of a nonterminal, it is also an instance of some production that has that nonterminal as
its left-hand side. Moreover, it has zero or more children, one for each symbol on the production's
right-hand side: each child is a Parse Node that is an instance of the corresponding symbol.
New Parse Nodes are instantiated for each invocation of the parser and never reused between parses even of
identical source text. Parse Nodes are considered the same Parse Node if and only
if they represent the same span of source text, are instances of the same grammar symbol, and resulted from
the same parser invocation.
Note 1
Parsing the same String multiple times will lead to different Parse Nodes. For example, consider:
let str = "1 + 1;";
eval(str);
eval(str);
Each call to eval converts the value of str into ECMAScript
source text and
performs an independent parse that creates its own separate tree of Parse Nodes. The trees are distinct
even though each parse operates upon a source text that was derived from the same String value.
Note 2
Parse Nodes are specification artefacts, and implementations are not required to
use an analogous data structure.
Productions of the syntactic grammar are distinguished by having just one colon “:” as punctuation.
The syntactic grammar as presented in clauses 13
through 16 is
not a complete account of which token sequences are accepted as a correct ECMAScript Script or Module. Certain
additional token sequences are also accepted, namely, those that would be described by the grammar if only
semicolons were added to the sequence in certain places (such as before line terminator characters).
Furthermore, certain token sequences that are described by the grammar are not considered acceptable if a
line terminator character appears in certain “awkward” places.
In certain cases, in order to avoid ambiguities, the syntactic grammar uses generalized productions that
permit token sequences that do not form a valid ECMAScript Script or Module. For example, this technique is
used for object literals and object destructuring patterns. In such cases a more restrictive
supplemental grammar is provided that further restricts the acceptable token sequences. Typically,
an early
error rule will then state that, in certain contexts, "P must cover an N", where P is a Parse Node (an instance of the
generalized production) and N is a nonterminal from the supplemental grammar. This means:
The sequence of tokens originally matched by P is parsed again using N as the
goal symbol. If
N takes grammatical parameters, then they are set to the same values used when P was
originally parsed.
If the sequence of tokens can be parsed as a single instance of N, with no tokens left over,
then:
We refer to that instance of N (a Parse Node, unique for a given P) as "the
N that is covered by P".
All Early Error rules for N and its derived productions also apply to the N
that is covered by P.
Otherwise (if the parse fails), it is an early Syntax Error.
5.1.5 Grammar Notation
5.1.5.1 Terminal Symbols
In the ECMAScript grammars, some terminal symbols are shown in fixed-width font. These are
to appear in a source text exactly as written. All terminal symbol code points specified in this way are
to be understood as the appropriate Unicode code points from the Basic Latin block, as opposed to any
similar-looking code points from other Unicode ranges. A code point in a terminal symbol cannot be
expressed by a \UnicodeEscapeSequence.
In grammars whose terminal symbols are individual Unicode code points (i.e., the lexical, RegExp, and
numeric string grammars), a contiguous run of multiple fixed-width code points appearing in a production
is a simple shorthand for the same sequence of code points, written as standalone terminal symbols.
In contrast, in the syntactic grammar, a contiguous run of fixed-width code points is a single terminal
symbol.
Terminal symbols come in two other forms:
In the lexical and RegExp grammars, Unicode code points without a conventional printed representation
are instead shown in the form "<ABBREV>" where "ABBREV" is a mnemonic for the code point or set of
code points. These forms are defined in Unicode
Format-Control Characters, White Space, and Line Terminators.
In the syntactic grammar, certain terminal symbols (e.g. IdentifierName and RegularExpressionLiteral)
are shown in italics, as they refer to the nonterminals of the same name in the lexical grammar.
5.1.5.2 Nonterminal Symbols and Productions
Nonterminal symbols are shown in italic type. The definition of a nonterminal (also called a
“production”) is introduced by the name of the nonterminal being defined followed by one or more colons.
(The number of colons indicates to which grammar the production belongs.) One or more alternative
right-hand sides for the nonterminal then follow on succeeding lines. For example, the syntactic
definition:
states that the nonterminal WhileStatement
represents the token while, followed by a left parenthesis token, followed by an Expression,
followed by a right parenthesis token, followed by a Statement. The occurrences of Expression and
Statement are
themselves nonterminals. As another example, the syntactic definition:
states that an ArgumentList may
represent either a single AssignmentExpression or an
ArgumentList,
followed by a comma, followed by an AssignmentExpression. This
definition of ArgumentList is
recursive, that is, it is defined in terms of itself. The result is that an ArgumentList may
contain any positive number of arguments, separated by commas, where each argument expression is an
AssignmentExpression. Such
recursive definitions of nonterminals are common.
5.1.5.3 Optional Symbols
The subscripted suffix “opt”, which may appear after a terminal or nonterminal, indicates an
optional symbol. The alternative containing the optional symbol actually specifies two right-hand sides,
one that omits the optional element and one that includes it. This means that:
so, in this example, the nonterminal ForStatement
actually has four alternative right-hand sides.
5.1.5.4 Grammatical Parameters
A production may be parameterized by a subscripted annotation of the form “[parameters]”,
which may appear as a suffix to the nonterminal symbol defined by the production. “parameters”
may be either a single name or a comma separated list of names. A parameterized production is shorthand
for a set of productions defining all combinations of the parameter names, preceded by an underscore,
appended to the parameterized nonterminal symbol. This means that:
Prefixing a parameter name with “?” on a right-hand side nonterminal reference makes that
parameter value dependent upon the occurrence of the parameter name on the reference to the current
production's left-hand side symbol. For example:
If a right-hand side alternative is prefixed with “[+parameter]” that alternative is only available if
the named parameter was used in referencing the production's nonterminal symbol. If a right-hand side
alternative is prefixed with “[~parameter]” that alternative is only available if the named parameter was
not used in referencing the production's nonterminal symbol. This means that:
When the words “one of” follow the colon(s) in a grammar definition, they signify that each of the
terminal symbols on the following line or lines is an alternative definition. For example, the lexical
grammar for ECMAScript contains the production:
If the phrase “[empty]” appears as the right-hand side of a production, it indicates that the
production's right-hand side contains no terminals or nonterminals.
5.1.5.7 Lookahead Restrictions
If the phrase “[lookahead = seq]” appears in the right-hand side of a production, it indicates
that the production may only be used if the token sequence seq is a prefix of the immediately
following input token sequence. Similarly, “[lookahead ∈ set]”, where set is a
finite
non-empty set of token sequences,
indicates that the production may only be used if some element of set is a prefix of the
immediately following token sequence. For convenience, the set can also be written as a nonterminal, in
which case it represents the set of all token sequences to which that nonterminal could expand. It is
considered an editorial error if the nonterminal could expand to infinitely many distinct token sequences.
These conditions may be negated. “[lookahead ≠ seq]” indicates that the containing production
may only be used if seq is not a prefix of the immediately following input token
sequence, and “[lookahead ∉ set]” indicates that the production may only be used if no
element of set is a prefix of the immediately following token sequence.
matches either the letter n followed by one or more decimal digits the first of which is
even, or a decimal digit not followed by another decimal digit.
Note that when these phrases are used in the syntactic grammar, it may not be possible to unambiguously
identify the immediately following token sequence because determining later tokens requires knowing which
lexical goal symbol to use at
later positions. As such, when these are used in the syntactic grammar, it is considered an editorial
error for a token sequence seq to appear in a lookahead restriction (including as part of a set
of sequences) if the choices of lexical goal symbols to use
could change whether or not seq would be a prefix of the resulting token sequence.
If the phrase “[no LineTerminator here]” appears in
the right-hand side of a production of the syntactic grammar, it indicates that the production is a
restricted production: it may not be used if a LineTerminator occurs in the
input stream at the indicated position. For example, the production:
indicates that the production may not be used if a LineTerminator occurs in the
script between the throw token and the Expression.
Unless the presence of a LineTerminator is forbidden by a
restricted production, any number of occurrences of LineTerminator may appear
between any two consecutive tokens in the stream of input elements without affecting the syntactic
acceptability of the script.
5.1.5.9 but not
The right-hand side of a production may specify that certain expansions are not permitted by using the
phrase “but not” and then indicating the expansions to be excluded. For example, the production:
means that the nonterminal Identifier may be
replaced by any sequence of code points that could replace IdentifierName provided that the
same sequence of code points could not replace ReservedWord.
5.1.5.10 Descriptive Phrases
Finally, a few nonterminal symbols are described by a descriptive phrase in sans-serif type in cases
where it would be impractical to list all the alternatives:
The specification often uses a numbered list to specify steps in an algorithm. These algorithms are used to
precisely specify the required semantics of ECMAScript language constructs. The algorithms are not intended to
imply the use of any specific implementation technique. In practice, there may be more efficient algorithms
available to implement a given feature.
Algorithms may be explicitly parameterized with an ordered, comma-separated sequence of alias names which may
be used within the algorithm steps to reference the argument passed in that position. Optional parameters are
denoted with surrounding brackets ([ , name ]) and are no different from required parameters within
algorithm steps. A rest parameter may appear at the end of a parameter list, denoted with leading ellipsis (,
...name). The rest parameter captures all of the arguments provided following the required and
optional parameters into a List. If there
are no such additional arguments, that List is empty.
Algorithm steps may be subdivided into sequential substeps. Substeps are indented and may themselves be
further divided into indented substeps. Outline numbering conventions are used to identify substeps with the
first level of substeps labelled with lowercase alphabetic characters and the second level of substeps
labelled with lowercase roman numerals. If more than three levels are required these rules repeat with the
fourth level using numeric labels. For example:
1. Top-level step
a. Substep.
b. Substep.
i. Subsubstep.
1. Subsubsubstep
a. Subsubsubsubstep
i. Subsubsubsubsubstep
A step or substep may be written as an “if” predicate that conditions its substeps. In this case, the
substeps are only applied if the predicate is true. If a step or substep begins with the word “else”, it is a
predicate that is the negation of the preceding “if” predicate step at the same level.
A step may specify the iterative application of its substeps.
A step that begins with “Assert:” asserts an invariant condition of its
algorithm. Such assertions are used to make explicit algorithmic invariants that would otherwise be implicit.
Such assertions add no additional semantic requirements and hence need not be checked by an implementation.
They are used simply to clarify algorithms.
Algorithm steps may declare named aliases for any value using the form “Let x be
someValue”. These aliases are reference-like in that both x and someValue
refer to the same underlying data and modifications to either are visible to both. Algorithm steps that want
to avoid this reference-like behaviour should explicitly make a copy of the right-hand side: “Let x
be a copy of someValue” creates a shallow copy of someValue.
Once declared, an alias may be referenced in any subsequent steps and must not be referenced from steps prior
to the alias's declaration. Aliases may be modified using the form “Set x to
someOtherValue”.
5.2.1 Abstract Operations
In order to facilitate their use in multiple parts of this specification, some algorithms, called abstract operations, are named and written in parameterized functional form so that
they may be referenced by name from within other algorithms. Abstract operations are typically referenced
using a functional application style such as OperationName(arg1, arg2). Some abstract
operations are treated as polymorphically dispatched methods of class-like specification abstractions. Such
method-like abstract operations are typically referenced using a method application style such as
someValue.OperationName(arg1, arg2).
5.2.2 Syntax-Directed Operations
A syntax-directed operation is a named
operation whose definition consists of algorithms, each of which is associated with one or more productions
from one of the ECMAScript grammars. A production that has multiple alternative definitions will typically
have a distinct algorithm for each alternative. When an algorithm is associated with a grammar production,
it may reference the terminal and nonterminal symbols of the production alternative as if they were
parameters of the algorithm. When used in this manner, nonterminal symbols refer to the actual alternative
definition that is matched when parsing the source text. The source text matched by a grammar
production or Parse Node derived from it is
the portion of the source text that starts at the beginning of the first terminal that participated in the
match and ends at the end of the last terminal that participated in the match.
When an algorithm is associated with a production alternative, the alternative is typically shown without
any “[ ]” grammar annotations. Such annotations should only affect the syntactic recognition of the
alternative and have no effect on the associated semantics for the alternative.
Syntax-directed operations are invoked with a parse node and, optionally, other parameters by using the
conventions on steps 1, 3, and 4 in the following
algorithm:
1. Let status be
SyntaxDirectedOperation of SomeNonTerminal.
2. Let someParseNode be the parse of some source text.
3. Perform SyntaxDirectedOperation
of someParseNode.
4. Perform SyntaxDirectedOperation
of someParseNode with argument "value".
Unless explicitly specified otherwise, all chain productions have an
implicit definition for every operation that might be applied to that production's left-hand side
nonterminal. The implicit definition simply reapplies the same operation with the same parameters, if any,
to the chain production's sole
right-hand side nonterminal and then returns the result. For example, assume that some algorithm has a step
of the form: “Return Evaluation of Block” and that there is a
production:
but the Evaluation operation does not
associate an algorithm with that production. In that case, the Evaluation operation implicitly
includes an association of the form:
The abstract operation Completion takes argument completionRecord (a Completion
Record) and returns a Completion
Record. It is used to emphasize that a Completion
Record is being returned. It performs the following steps when called:
Similarly, prefix ! is used to indicate that the following invocation of an abstract or
syntax-directed
operation will never return an abrupt
completion and that the resulting Completion
Record's [[Value]] field should be used in place of the return
value of the operation. For example, the step:
1. Perform ! SyntaxDirectedOperation of
NonTerminal.
5.2.3.5 Implicit Normal Completion
In algorithms within abstract
operations which are declared to return a Completion
Record, and within all built-in functions, the returned value is first passed to
NormalCompletion, and the
result is used instead. This rule does not apply within the Completion algorithm or when the
value being returned is clearly marked as a Completion
Record in that step; these cases are:
when the result of constructing a Completion
Record is directly returned
It is an editorial error if a Completion
Record is returned from such an abstract operation through any other means. For
example, within these abstract
operations,
Note that, through the ReturnIfAbrupt expansion, the
following example is allowed, as within the expanded steps, the result of applying Completion is returned directly
in the abrupt case and the implicit NormalCompletion application
occurs after unwrapping in the normal case.
1. Return ? completion.
The following example would be an editorial error because a Completion
Record is being returned without being annotated in that step.
Context-free grammars are not sufficiently powerful to express all the rules that define whether a stream
of input elements form a valid ECMAScript Script or Module that may be evaluated. In some
situations additional rules are needed that may be expressed using either ECMAScript algorithm conventions
or prose requirements. Such rules are always associated with a production of a grammar and are called the
static semantics of the production.
Static Semantic Rules have names and typically are defined using an algorithm. Named Static Semantic Rules
are associated with grammar productions and a production that has multiple alternative definitions will
typically have for each alternative a distinct algorithm for each applicable named static semantic rule.
A special kind of static semantic rule is an Early Error
Rule. Early error rules define early
error conditions (see clause 17) that
are associated with specific grammar productions. Evaluation of most early
error rules are not explicitly invoked within the algorithms of this specification. A
conforming implementation must, prior to the first evaluation of a Script or Module, validate all of the early
error rules of the productions used to parse that Script or Module. If any of the early
error rules are violated the Script or Module is invalid and cannot be evaluated.
5.2.5 Mathematical Operations
This specification makes reference to these kinds of numeric values:
Mathematical values: Arbitrary real numbers, used as the default numeric type.
In the language of this specification, numerical values are distinguished among different numeric kinds
using subscript suffixes. The subscript 𝔽 refers to Numbers, and the subscript ℤ
refers to BigInts. Numeric values without a subscript suffix refer to mathematical
values.
Numeric operators such as +, ×, =, and ≥ refer to those operations as determined by the type of the
operands. When applied to mathematical values, the
operators refer to the usual mathematical operations. When applied to extended mathematical
values, the operators refer to the usual mathematical operations over the extended real
numbers; indeterminate forms are not defined and their use in this specification should be considered an
editorial error. When applied to Numbers, the operators refer to the relevant operations within IEEE
754-2019. When applied to BigInts, the operators refer to the usual mathematical
operations applied to the mathematical value of the
BigInt.
In general, when this specification refers to a numerical value, such as in the phrase, "the length of
y" or "the integer represented by the four hexadecimal
digits ...", without explicitly specifying a numeric kind, the phrase refers to a mathematical value. Phrases which
refer to a Number or a BigInt value are explicitly annotated as such; for example, "the Number
value for the number of code points in …" or "the BigInt value for
…".
Numeric operators applied to mixed-type operands (such as a Number and a mathematical value) are not
defined and should be considered an editorial error in this specification.
This specification denotes most numeric values in base 10; it also uses numeric values of the form 0x
followed by digits 0-9 or A-F as base-16 values.
When the term integer is used in this specification, it refers to a mathematical value which is in
the set of integers, unless otherwise stated. When the
term integral Number is used in
this specification, it refers to a Number value whose mathematical value is in the set
of integers.
The mathematical function abs(x)
produces the absolute value of x, which is -x if
x < 0 and otherwise is x itself.
The mathematical function min(x1, x2,
… , xN) produces the mathematically smallest of x1 through xN. The
mathematical function max(x1, x2, ...,
xN) produces the mathematically largest of x1 through xN. The domain
and range of these mathematical functions are the extended mathematical
values.
The notation “x modulo
y” (y must be finite and non-zero) computes a value
k of the same sign as y (or zero) such that abs(k)
< abs(y) and x -
k = q × y for some integerq.
The phrase "the result of clamping x between
lower and upper" (where x is an extended mathematical
value and lower and upper are mathematical
values such that lower ≤ upper) produces lower if
x < lower, produces upper if x > upper, and
otherwise produces x.
The mathematical function floor(x)
produces the largest integer (closest to +∞) that is not larger
than x.
The mathematical function truncate(x) removes the fractional part of x by rounding
towards zero, producing -floor(-x) if
x < 0 and otherwise producing floor(x).
Mathematical functions min, max, abs, floor, and truncate are not defined
for Numbers
and BigInts, and any usage of those methods that have non-mathematical
value arguments would be an editorial error in this specification.
An interval from lower bound a to upper bound
b is a possibly-infinite, possibly-empty set of numeric values of the same numeric type. Each
bound will be described as either inclusive or exclusive, but not both. There are four kinds of intervals,
as follows:
An interval from a (inclusive) to
b (inclusive), also called an inclusive
interval from a to b, includes all values x of the same numeric
type such that a ≤ x ≤ b, and no others.
An interval from a (inclusive) to
b (exclusive) includes all values x of the same numeric type such that a
≤ x < b, and no others.
An interval from a (exclusive) to
b (inclusive) includes all values x of the same numeric type such that a
< x ≤ b, and no others.
An interval from a (exclusive) to
b (exclusive) includes all values x of the same numeric type such that a
< x < b, and no others.
For example, the interval from 1 (inclusive) to 2
(exclusive) consists of all mathematical values between 1 and
2, including 1 and not including 2. For the purpose of defining intervals,
-0𝔽 <
+0𝔽, so, for example, an inclusive
interval with a lower bound of +0𝔽 includes
+0𝔽 but not -0𝔽. NaN is
never included in an interval.
5.2.6 Value Notation
In this specification, ECMAScript language
values are displayed in bold. Examples include
null, true, or "hello". These are distinguished
from ECMAScript source text such as
Function.prototype.apply or let n = 42;.
5.2.7 Identity
In this specification, both specification values and ECMAScript language
values are compared for equality. When comparing for equality, values fall into one of
two categories. Values without
identity are equal to other values without identity if all of their innate characteristics are the
same — characteristics such as the magnitude of an integer or the length of a sequence. Values
without identity may be manifest without prior reference by fully describing their characteristics. In
contrast, each value with identity is unique and
therefore only equal to itself. Values with identity are like values without identity but with an additional
unguessable, unchangeable, universally-unique characteristic called identity. References to
existing values with identity cannot be manifest simply by describing them, as the identity itself is
indescribable; instead, references to these values must be explicitly passed from one place to another. Some
values with identity are mutable and therefore can have their characteristics (except their identity)
changed in-place, causing all holders of the value to observe the new characteristics. A value without
identity is never equal to a value with identity.
From the perspective of this specification, the word “is” is used to compare two values for equality, as in
“If bool is true, then ...”, and the word “contains” is used to search for a
value inside lists using equality comparisons, as in "If list contains a Recordr such that r.[[Foo]] is true, then ...".
The specification identity of values determines the result of these comparisons and is axiomatic in
this specification.
From the perspective of the ECMAScript language, language values are compared for equality using the
SameValue abstract operation and the
abstract
operations it transitively calls. The algorithms of these comparison abstract
operations determine language identity of ECMAScript language
values.
Algorithms within this specification manipulate values each of which has an associated type. The possible value
types are exactly those defined in this clause. Types are further subclassified into ECMAScript language types
and specification types.
Within this specification, the notation “Type(x)” is used as shorthand for “the type of
x” where “type” refers to the ECMAScript language and specification types defined in this clause.
6.1 ECMAScript Language Types
An ECMAScript language type corresponds to
values that are directly manipulated by an ECMAScript programmer using the ECMAScript language. The ECMAScript
language types are Undefined, Null, Boolean, String, Symbol, Number, BigInt, and Object. An ECMAScript language value is a value that is
characterized by an ECMAScript language type.
6.1.1 The Undefined Type
The Undefined type has exactly one value, called undefined. Any variable that has not
been assigned a value has the value undefined.
6.1.2 The Null Type
The Null type has exactly one value, called null.
6.1.3 The Boolean Type
The Boolean type represents a logical
entity having two values, called true and false.
6.1.4 The String Type
The String type is the set of all ordered
sequences of zero or more 16-bit unsigned integer values (“elements”) up to a maximum
length of 2**53 - 1 elements. The String type is generally used
to represent textual data in a running ECMAScript program, in which case each element in the String is
treated as a UTF-16 code unit value. Each element is regarded as occupying a position within the sequence.
These positions are indexed with non-negative integers. The first element (if any) is at
index 0, the next element (if any) at index 1, and so on. The length of a String is the number of elements
(i.e., 16-bit values) within it. The empty String has length zero and therefore contains no elements.
ECMAScript operations that do not interpret String contents apply no further semantics. Operations that do
interpret String values treat each element as a single UTF-16 code unit. However, ECMAScript does not
restrict the value of or relationships between these code units, so operations that further interpret String
contents as sequences of Unicode code points encoded in UTF-16 must account for ill-formed subsequences.
Such operations apply special treatment to every code unit with a numeric value in the inclusive
interval from 0xD800 to
0xDBFF (defined by the Unicode Standard as a leading surrogate, or more formally as a high-surrogate code unit) and every code unit
with a numeric value in the inclusive interval from 0xDC00 to
0xDFFF (defined as a trailing
surrogate, or more formally as a low-surrogate code unit) using the following
rules:
A sequence of two code units, where the first code unit c1 is a leading
surrogate and the second
code unit c2 a trailing surrogate, is a surrogate pair and is interpreted as
a code point with the value (c1 - 0xD800) × 0x400 + (c2 - 0xDC00) + 0x10000. (See
11.1.3)
The function String.prototype.normalize (see 22.1.3.15)
can be
used to explicitly normalize a String value. String.prototype.localeCompare (see 22.1.3.12)
internally normalizes String values, but no other operations implicitly normalize the strings upon which
they operate. Operation results are not language- and/or locale-sensitive unless stated otherwise.
Note
The rationale behind this design was to keep the implementation of Strings as simple and
high-performing as possible. If ECMAScript source text is in
Normalized Form C, string literals are guaranteed to also be normalized, as long as they do not contain
any Unicode escape sequences.
In this specification, the phrase "the string-concatenation of A, B, ..." (where each argument is a
String value, a code unit, or a sequence of code units) denotes the String value whose sequence of code
units is the concatenation of the code units (in order) of each of the arguments (in order).
The phrase "the substring of S from
inclusiveStart to exclusiveEnd" (where S is a String value or a sequence of
code units and inclusiveStart and exclusiveEnd are integers) denotes the String
value consisting of the consecutive code units of S beginning at index inclusiveStart
and ending immediately before index exclusiveEnd (which is the empty String when
inclusiveStart = exclusiveEnd). If the "to" suffix is omitted, the length of
S is used as the value of exclusiveEnd.
The phrase "the ASCII word characters" denotes the
following String value, which consists solely of every letter and number in the Unicode Basic Latin block
along with U+005F (LOW LINE): "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789_".
For historical reasons, it has significance to various algorithms.
The abstract operation StringIndexOf takes arguments string (a String), searchValue
(a String), and fromIndex (a non-negative integer) and returns an integer.
It performs the following steps when called:
1. Let len be the length of string.
2. If searchValue is the empty String and
fromIndex ≤ len, return fromIndex.
3. Let searchLen be the length of searchValue.
4. For each integeri such that
fromIndex ≤ i ≤ len - searchLen, in ascending order, do
a. Let candidate be the substring of
string from i to i + searchLen.
b. If candidate is searchValue, return
i.
5. Return -1.
Note 1
If searchValue is the empty String and fromIndex ≤ the length of
string, this algorithm returns fromIndex. The empty String is effectively found
at every position within a string, including after the last code unit.
Note 2
This algorithm always returns -1 if fromIndex + the length of searchValue >
the length of string.
6.1.5 The Symbol Type
The Symbol type is the set of all
non-String values that may be used as the key of an Object property (6.1.7).
Each possible Symbol value is unique and immutable.
Each Symbol value immutably holds an associated value called [[Description]] that
is either undefined or a String value.
6.1.5.1 Well-Known Symbols
Well-known symbols are built-in Symbol values that are explicitly referenced by algorithms of this
specification. They are typically used as the keys of properties whose values serve as extension points of
a specification algorithm. Unless otherwise specified, well-known symbols values are shared by all
realms
(9.3).
Within this specification a well-known symbol is referred to by using a notation of the form @@name,
where “name” is one of the values listed in Table 1.
Table 1: Well-known Symbols
Specification Name
[[Description]]
Value and Purpose
@@asyncIterator
"Symbol.asyncIterator"
A method that returns the default AsyncIterator for an object. Called by the semantics of the
for-await-of statement.
@@hasInstance
"Symbol.hasInstance"
A method that determines if a constructor object
recognizes an object as one of the constructor's instances.
Called by the semantics of the instanceof operator.
@@isConcatSpreadable
"Symbol.isConcatSpreadable"
A Boolean valued property that if true indicates that an object should be flattened to its array
elements by Array.prototype.concat.
@@iterator
"Symbol.iterator"
A method that returns the default Iterator for an object. Called by the semantics of the for-of
statement.
@@match
"Symbol.match"
A regular expression method that matches the regular expression against a string. Called by the
String.prototype.match
method.
@@matchAll
"Symbol.matchAll"
A regular expression method that returns an iterator, that yields matches of the regular
expression against a string. Called by the String.prototype.matchAll
method.
@@replace
"Symbol.replace"
A regular expression method that replaces matched substrings of a string. Called by the
String.prototype.replace
method.
@@search
"Symbol.search"
A regular expression method that returns the index within a string that matches the regular
expression. Called by the String.prototype.search
method.
@@species
"Symbol.species"
A function valued property that is the constructor function that is
used to create derived objects.
@@split
"Symbol.split"
A regular expression method that splits a string at the indices that match the regular
expression. Called by the String.prototype.split
method.
@@toPrimitive
"Symbol.toPrimitive"
A method that converts an object to a corresponding primitive value. Called by the ToPrimitive abstract
operation.
@@toStringTag
"Symbol.toStringTag"
A String valued property that is used in the creation of the default string description of an
object. Accessed by the built-in method Object.prototype.toString.
@@unscopables
"Symbol.unscopables"
An object valued property whose own and inherited property names are property names that are
excluded from the with environment bindings of the associated object.
6.1.6 Numeric Types
ECMAScript has two built-in numeric types: Number and BigInt. The following abstract
operations are defined over these numeric types. The "Result" column shows the return
type, along with an indication if it is possible for some invocations of the operation to return an
abrupt
completion.
Because the numeric types are in general not convertible without loss of precision or truncation, the
ECMAScript language provides no implicit conversion among these types. Programmers must explicitly call
Number and BigInt functions to convert among types when calling a function which
requires another type.
Note
The first and subsequent editions of ECMAScript have provided, for certain operators, implicit numeric
conversions that could lose precision or truncate. These legacy implicit
conversions are maintained for backward compatibility, but not provided for BigInt in order to minimize
opportunity for programmer error, and to leave open the option of generalized value types in a
future edition.
6.1.6.1 The Number Type
The Number type has exactly
18,437,736,874,454,810,627 (that is, 2**64 - 2**53 +
3) values, representing the double-precision 64-bit format IEEE 754-2019
values as specified in the IEEE Standard for Binary Floating-Point Arithmetic, except that the
9,007,199,254,740,990 (that is, 2**53 -
2) distinct “Not-a-Number” values of the IEEE Standard are represented in ECMAScript as a
single special NaN value. (Note that the NaN value is produced by
the program expression NaN.) In some implementations, external code might be able to detect a
difference between various Not-a-Number values, but such behaviour is implementation-defined; to
ECMAScript code, all NaN values are indistinguishable from each other.
Note
The bit pattern that might be observed in an ArrayBuffer (see 25.1) or a
SharedArrayBuffer (see 25.2) after a
Number value has been stored into it is not necessarily the same as the internal representation of
that Number value used by the ECMAScript implementation.
There are two other special values, called positive Infinity and negative
Infinity. For brevity, these values are also referred to for expository purposes by the
symbols +∞𝔽 and -∞𝔽, respectively. (Note
that these two infinite Number values are produced by the program expressions +Infinity (or
simply Infinity) and -Infinity.)
The other 18,437,736,874,454,810,624 (that is, 2**64 - 2**53) values are called the finite numbers. Half of these are positive numbers and half are negative numbers;
for every finite positive Number value there is a
corresponding negative value having the same magnitude.
Note that there is both a positive zero and a negative zero. For
brevity, these values are also referred to for expository purposes by the symbols
+0𝔽 and -0𝔽, respectively. (Note that these
two different zero Number values are produced by the program expressions +0 (or simply
0) and -0.)
The 18,437,736,874,454,810,622 (that is, 2**64 - 2**53 -
2) finite non-zero values are of two kinds:
18,428,729,675,200,069,632 (that is, 2**64 - 2**54) of them are normalized, having the form
s × m × 2**e
where s is 1 or -1, m is an integer in the interval from 2**52 (inclusive) to 2**53 (exclusive), and e is an integer in the inclusive interval from -1074
to 971.
The remaining 9,007,199,254,740,990 (that is, 2**53 - 2) values are denormalized, having the form
s × m × 2**e
where s is 1 or -1, m is an integer in the interval from 0
(exclusive) to 2**52 (exclusive), and e is -1074.
Note that all the positive and negative integers whose magnitude is no greater
than 2**53 are representable in the Number type. The integer 0
has two representations in the Number type: +0𝔽 and
-0𝔽.
A finite number has an odd
significand if it is non-zero and the integerm used to express it
(in one of the two forms shown above) is odd. Otherwise, it has an even significand.
In this specification, the phrase “the Number value for x” where x
represents an exact real mathematical quantity (which might even be an irrational number such as π) means
a Number value chosen in the following manner. Consider the set of all finite values of the Number
type, with -0𝔽 removed and with two additional values added to it that are
not representable in the Number type, namely 2**1024 (which is
+1 × 2**53 × 2**971) and -2**1024 (which is -1 × 2**53 × 2**971). Choose the member of this set that is closest
in value to x. If two values of the set are equally close, then the one with an even
significand is chosen; for this purpose, the two extra values 2**1024 and -2**1024 are considered to have even significands.
Finally, if 2**1024 was chosen, replace it with
+∞𝔽; if -2**1024 was chosen, replace it with
-∞𝔽; if +0𝔽 was chosen, replace it with
-0𝔽 if and only if x < 0; any other chosen value is used
unchanged. The result is the Number value forx.
(This procedure corresponds exactly to the behaviour of the IEEE 754-2019
roundTiesToEven mode.)
Some ECMAScript operators deal only with integers in specific ranges such as the
inclusive interval from
-2**31 to 2**31 - 1 or the inclusive interval from 0 to
2**16 - 1. These operators
accept any value of the Number type but first convert each such value to an integer value in the
expected range. See the descriptions of the numeric conversion operations in 7.1.
6.1.6.1.1 Number::unaryMinus ( x )
The abstract operation Number::unaryMinus takes argument x (a Number) and returns a Number.
It performs the following steps when called:
1. If x is NaN, return
NaN.
2. Return the result of negating x; that is, compute a
Number with the same magnitude but opposite sign.
6.1.6.1.2 Number::bitwiseNOT ( x )
The abstract operation Number::bitwiseNOT takes argument x (a Number) and returns an
integral Number. It performs the
following steps when called:
2. Return the result of applying bitwise complement to
oldValue. The mathematical value of
the result is exactly representable as a 32-bit two's complement bit string.
6.1.6.1.3 Number::exponentiate ( base, exponent )
The abstract operation Number::exponentiate takes arguments base (a Number) and
exponent (a Number) and returns a Number. It returns an implementation-approximated
value representing the result of raising base to the exponent power. It performs
the following steps when called:
1. If exponent is NaN, return
NaN.
2. If exponent is either
+0𝔽 or -0𝔽, return
1𝔽.
3. If base is NaN, return
NaN.
4. If base is +∞𝔽, then
a. If exponent >
+0𝔽, return +∞𝔽. Otherwise, return
+0𝔽.
5. If base is -∞𝔽, then
a. If exponent >
+0𝔽, then
i. If exponent is an odd integral Number,
return -∞𝔽. Otherwise, return
+∞𝔽.
b. Else,
i. If exponent is an odd integral Number,
return -0𝔽. Otherwise, return
+0𝔽.
6. If base is +0𝔽, then
a. If exponent >
+0𝔽, return +0𝔽. Otherwise, return
+∞𝔽.
7. If base is -0𝔽, then
a. If exponent >
+0𝔽, then
i. If exponent is an odd integral Number,
return -0𝔽. Otherwise, return
+0𝔽.
b. Else,
i. If exponent is an odd integral Number,
return -∞𝔽. Otherwise, return
+∞𝔽.
8. Assert: base is finite
and is neither +0𝔽 nor -0𝔽.
11. Assert: exponent is
finite and is neither
+0𝔽 nor -0𝔽.
12. If base < -0𝔽 and
exponent is not an integral Number, return
NaN.
13. Return an implementation-approximated
Number value representing the result of raising ℝ(base) to the
ℝ(exponent) power.
Note
The result of base**exponent when base is
1𝔽 or -1𝔽 and exponent is
+∞𝔽 or -∞𝔽, or when base is
1𝔽 and exponent is NaN, differs from
IEEE 754-2019. The first
edition of ECMAScript specified a result of NaN for this operation, whereas later
revisions of IEEE 754 specified 1𝔽. The historical ECMAScript
behaviour is preserved for compatibility reasons.
6.1.6.1.4 Number::multiply ( x, y )
The abstract operation Number::multiply takes arguments x (a Number) and y (a
Number) and returns a Number. It performs multiplication according to the rules of IEEE
754-2019 binary double-precision arithmetic, producing the product of x
and y. It performs the following steps when called:
Finite-precision multiplication is
commutative, but not always associative.
6.1.6.1.5 Number::divide ( x, y )
The abstract operation Number::divide takes arguments x (a Number) and y (a
Number) and returns a Number. It performs division according to the rules of IEEE
754-2019 binary double-precision arithmetic, producing the quotient of x
and y where x is the dividend and y is the divisor. It performs the
following steps when called:
1. If x is NaN or y is
NaN, return NaN.
2. If x is either +∞𝔽 or
-∞𝔽, then
a. If y is either
+∞𝔽 or -∞𝔽, return
NaN.
b. If y is +0𝔽 or
y > +0𝔽, return x.
c. Return -x.
3. If y is +∞𝔽, then
a. If x is +0𝔽 or
x > +0𝔽, return +0𝔽.
Otherwise, return -0𝔽.
4. If y is -∞𝔽, then
a. If x is +0𝔽 or
x > +0𝔽, return -0𝔽.
Otherwise, return +0𝔽.
The abstract operation Number::remainder takes arguments n (a Number) and d (a
Number) and returns a Number. It yields the remainder from an implied division of its operands where
n is the dividend and d is the divisor. It performs the following steps when
called:
In C and C++, the remainder operator accepts only integral operands; in ECMAScript, it also accepts
floating-point operands.
Note 2
The result of a floating-point remainder operation as computed by the
% operator is not the same as the “remainder” operation defined by IEEE
754-2019. The IEEE 754-2019 “remainder”
operation computes the remainder from a rounding division, not a truncating division, and so its
behaviour is not analogous to that of the usual integer remainder operator.
Instead the ECMAScript language defines % on floating-point operations to behave in a
manner analogous to that of the Java integer remainder operator; this may
be compared with the C library function fmod.
6.1.6.1.7 Number::add ( x, y )
The abstract operation Number::add takes arguments x (a Number) and y (a Number)
and returns a Number. It performs addition according to the rules of IEEE 754-2019
binary double-precision arithmetic, producing the sum of its arguments. It performs the following steps
when called:
Finite-precision addition is
commutative, but not always associative.
6.1.6.1.8 Number::subtract ( x, y )
The abstract operation Number::subtract takes arguments x (a Number) and y (a
Number) and returns a Number. It performs subtraction, producing the difference of its operands;
x is the minuend and y is the subtrahend. It performs the following steps when
called:
It is always the case that x - y produces the same result as x + (-y).
6.1.6.1.9 Number::leftShift ( x, y )
The abstract operation Number::leftShift takes arguments x (a Number) and y (a
Number) and returns an integral Number. It performs the
following steps when called:
4. Return the result of left shifting lnum by
shiftCount bits. The mathematical value of
the result is exactly representable as a 32-bit two's complement bit string.
6.1.6.1.10 Number::signedRightShift ( x, y )
The abstract operation Number::signedRightShift takes arguments x (a Number) and
y (a Number) and returns an integral Number. It performs the
following steps when called:
4. Return the result of performing a sign-extending right shift of
lnum by shiftCount bits. The most significant bit is propagated. The mathematical value of
the result is exactly representable as a 32-bit two's complement bit string.
6.1.6.1.11 Number::unsignedRightShift ( x, y )
The abstract operation Number::unsignedRightShift takes arguments x (a Number) and
y (a Number) and returns an integral Number. It performs the
following steps when called:
4. Return the result of performing a zero-filling right shift of
lnum by shiftCount bits. Vacated bits are filled with zero. The mathematical value of
the result is exactly representable as a 32-bit unsigned bit string.
6.1.6.1.12 Number::lessThan ( x, y )
The abstract operation Number::lessThan takes arguments x (a Number) and y (a
Number) and returns a Boolean or undefined. It performs the following steps when
called:
11. If ℝ(x) < ℝ(y), return
true. Otherwise, return false.
6.1.6.1.13 Number::equal ( x, y )
The abstract operation Number::equal takes arguments x (a Number) and y (a
Number) and returns a Boolean. It performs the following steps when called:
1. If x is NaN, return
false.
2. If y is NaN, return
false.
3. If x is y, return
true.
4. If x is +0𝔽 and
y is -0𝔽, return true.
5. If x is -0𝔽 and
y is +0𝔽, return true.
6. Return false.
6.1.6.1.14 Number::sameValue ( x, y )
The abstract operation Number::sameValue takes arguments x (a Number) and y (a
Number) and returns a Boolean. It performs the following steps when called:
1. If x is NaN and y is
NaN, return true.
2. If x is +0𝔽 and
y is -0𝔽, return false.
3. If x is -0𝔽 and
y is +0𝔽, return false.
4. If x is y, return
true.
5. Return false.
6.1.6.1.15 Number::sameValueZero ( x, y )
The abstract operation Number::sameValueZero takes arguments x (a Number) and y
(a Number) and returns a Boolean. It performs the following steps when called:
1. If x is NaN and y is
NaN, return true.
2. If x is +0𝔽 and
y is -0𝔽, return true.
3. If x is -0𝔽 and
y is +0𝔽, return true.
4. If x is y, return
true.
5. Return false.
6.1.6.1.16 NumberBitwiseOp ( op, x, y )
The abstract operation NumberBitwiseOp takes arguments op (&,
^, or |), x (a Number), and y (a Number) and returns an
integral Number. It performs the
following steps when called:
b. Let result be the result of applying the bitwise
inclusive OR operation to lbits and rbits.
8. Return the Number value for the
integer represented by the 32-bit
two's complement bit string result.
6.1.6.1.17 Number::bitwiseAND ( x, y )
The abstract operation Number::bitwiseAND takes arguments x (a Number) and y (a
Number) and returns an integral Number. It performs the
following steps when called:
The abstract operation Number::bitwiseXOR takes arguments x (a Number) and y (a
Number) and returns an integral Number. It performs the
following steps when called:
The abstract operation Number::bitwiseOR takes arguments x (a Number) and y (a
Number) and returns an integral Number. It performs the
following steps when called:
The abstract operation Number::toString takes arguments x (a Number) and radix
(an integer in the inclusive interval from 2 to
36) and returns a String. It represents x as a String using a positional numeral system with
radix radix. The digits used in the representation of a number using radix r are
taken from the first r code units of
"0123456789abcdefghijklmnopqrstuvwxyz" in order. The representation of numbers with
magnitude greater than or equal to 1𝔽 never includes leading zeroes. It
performs the following steps when called:
5. Let n,
k, and s be integers such that k ≥ 1,
radix**(k - 1) ≤ s < radix**k, 𝔽(s ×
radix**(n - k)) is x, and k is as small as possible. Note that
k is the number of digits in the representation of s using radix
radix, that s is not divisible by radix, and that the least
significant digit of s is not necessarily uniquely determined by these criteria.
the code unit of the most significant digit of the decimal representation of s
the code unit 0x002E (FULL STOP)
the code units of the remaining k - 1 digits of the decimal representation of
s
the code unit 0x0065 (LATIN SMALL LETTER E)
exponentSign
the code units of the decimal representation of abs(n - 1)
Note 1
The following observations may be useful as guidelines for implementations, but are not part of the
normative requirements of this Standard:
If x is any Number value other than -0𝔽, then ToNumber(ToString(x)) is x.
The least significant digit of s is not always uniquely determined by the requirements listed in
step 5.
Note 2
For implementations that provide more accurate conversions than required by the rules above, it is
recommended that the following alternative version of step 5
be used as a guideline:
5. Let n, k, and s be
integers such that k
≥ 1, radix**(k - 1) ≤ s < radix**k, 𝔽(s ×
radix**(n - k)) is x, and k is as small as possible. If
there are multiple possibilities for s, choose the value of s for which
s × radix**(n -
k) is closest in value to ℝ(x).
If there are
two such possible values of s, choose the one that is even. Note that k is
the number of digits in the representation of s using radix radix and that
s is not divisible by radix.
Note 3
Implementers of ECMAScript may find useful the paper and code written by David M. Gay for
binary-to-decimal conversion of floating-point numbers:
The BigInt type represents an integer
value. The value may be any size and is not limited to a particular bit-width. Generally, where not
otherwise noted, operations are designed to return exact mathematically-based answers. For binary
operations, BigInts act as two's complement binary strings, with negative numbers treated as having bits
set infinitely to the left.
6.1.6.2.1 BigInt::unaryMinus ( x )
The abstract operation BigInt::unaryMinus takes argument x (a BigInt) and returns a BigInt.
It performs the following steps when called:
1. If x is 0ℤ, return
0ℤ.
2. Return -x.
6.1.6.2.2 BigInt::bitwiseNOT ( x )
The abstract operation BigInt::bitwiseNOT takes argument x (a BigInt) and returns a BigInt.
It returns the one's complement of x. It performs the following steps when called:
1. Return -x - 1ℤ.
6.1.6.2.3 BigInt::exponentiate ( base, exponent )
The abstract operation BigInt::exponentiate takes arguments base (a BigInt) and
exponent (a BigInt) and returns either a normal completion
containing a BigInt or a throw
completion. It performs the following steps when called:
1. If exponent < 0ℤ,
throw a RangeError exception.
2. If base is 0ℤ and
exponent is 0ℤ, return 1ℤ.
3. Return base raised to the power exponent.
6.1.6.2.4 BigInt::multiply ( x, y )
The abstract operation BigInt::multiply takes arguments x (a BigInt) and y (a
BigInt) and returns a BigInt. It performs the following steps when called:
1. Return x × y.
Note
Even if the result has a much larger bit width than the input, the exact
mathematical answer is given.
6.1.6.2.5 BigInt::divide ( x, y )
The abstract operation BigInt::divide takes arguments x (a BigInt) and y (a
BigInt) and returns either a normal
completion containing a BigInt or a throw
completion. It performs the following steps when called:
The abstract operation BigInt::remainder takes arguments n (a BigInt) and d (a
BigInt) and returns either a normal
completion containing a BigInt or a throw
completion. It performs the following steps when called:
The sign of the result is the sign of the dividend.
6.1.6.2.7 BigInt::add ( x, y )
The abstract operation BigInt::add takes arguments x (a BigInt) and y (a BigInt)
and returns a BigInt. It performs the following steps when called:
1. Return x + y.
6.1.6.2.8 BigInt::subtract ( x, y )
The abstract operation BigInt::subtract takes arguments x (a BigInt) and y (a
BigInt) and returns a BigInt. It performs the following steps when called:
1. Return x - y.
6.1.6.2.9 BigInt::leftShift ( x, y )
The abstract operation BigInt::leftShift takes arguments x (a BigInt) and y (a
BigInt) and returns a BigInt. It performs the following steps when called:
Semantics here should be equivalent to a bitwise shift, treating the BigInt
as an infinite length string of binary two's complement digits.
6.1.6.2.10 BigInt::signedRightShift ( x, y )
The abstract operation BigInt::signedRightShift takes arguments x (a BigInt) and
y (a BigInt) and returns a BigInt. It performs the following steps when called:
The abstract operation BigInt::unsignedRightShift takes arguments x (a BigInt) and
y (a BigInt) and returns a throw
completion. It performs the following steps when called:
1. Throw a TypeError exception.
6.1.6.2.12 BigInt::lessThan ( x, y )
The abstract operation BigInt::lessThan takes arguments x (a BigInt) and y (a
BigInt) and returns a Boolean. It performs the following steps when called:
1. If ℝ(x) < ℝ(y), return
true; otherwise return false.
6.1.6.2.13 BigInt::equal ( x, y )
The abstract operation BigInt::equal takes arguments x (a BigInt) and y (a
BigInt) and returns a Boolean. It performs the following steps when called:
1. If ℝ(x) = ℝ(y),
return
true; otherwise return false.
6.1.6.2.14 BinaryAnd ( x, y )
The abstract operation BinaryAnd takes arguments x (0 or 1) and y (0 or 1) and
returns 0 or 1. It performs the following steps when called:
1. If x = 1 and y = 1, return 1.
2. Else, return 0.
6.1.6.2.15 BinaryOr ( x, y )
The abstract operation BinaryOr takes arguments x (0 or 1) and y (0 or 1) and
returns 0 or 1. It performs the following steps when called:
1. If x = 1 or y = 1, return 1.
2. Else, return 0.
6.1.6.2.16 BinaryXor ( x, y )
The abstract operation BinaryXor takes arguments x (0 or 1) and y (0 or 1) and
returns 0 or 1. It performs the following steps when called:
1. If x = 1 and y = 0, return 1.
2. Else if x = 0 and y = 1, return 1.
3. Else, return 0.
6.1.6.2.17 BigIntBitwiseOp ( op, x, y )
The abstract operation BigIntBitwiseOp takes arguments op (&,
^, or |), x (a BigInt), and y (a BigInt) and returns a
BigInt. It performs the following steps when called:
The abstract operation BigInt::bitwiseAND takes arguments x (a BigInt) and y (a
BigInt) and returns a BigInt. It performs the following steps when called:
The abstract operation BigInt::bitwiseXOR takes arguments x (a BigInt) and y (a
BigInt) and returns a BigInt. It performs the following steps when called:
The abstract operation BigInt::bitwiseOR takes arguments x (a BigInt) and y (a
BigInt) and returns a BigInt. It performs the following steps when called:
The abstract operation BigInt::toString takes arguments x (a BigInt) and radix
(an integer in the inclusive interval from 2 to
36) and returns a String. It represents x as a String using a positional numeral system with
radix radix. The digits used in the representation of a BigInt using radix r are
taken from the first r code units of
"0123456789abcdefghijklmnopqrstuvwxyz" in order. The representation of BigInts other
than 0ℤ never includes leading zeroes. It performs the following steps
when called:
2. Return the String value consisting of the representation of
x using radix radix.
6.1.7 The Object Type
Each instance of the Object type, also
referred to simply as “an Object”, represents a collection of properties. Each property is either a data
property, or an accessor property:
A data property associates a key value with an
ECMAScript language
value and a set of Boolean attributes.
An accessor property associates a key value with
one or two accessor functions, and a set of Boolean attributes. The accessor functions are used to store
or retrieve an ECMAScript language
value that is associated with the property.
The properties of an object are uniquely identified using property keys. A property key is either a String or a Symbol. All Strings and Symbols, including the
empty String, are valid as property keys. A property name is a
property key that is a
String.
Property keys are used to access properties and their values. There are two kinds of access for properties:
get and set, corresponding to value retrieval and assignment, respectively. The properties
accessible via get and set access includes both own properties that are a direct part of an object
and inherited properties which are provided by another associated object via a property inheritance
relationship. Inherited properties may be either own or inherited properties of the associated object. Each
own property of an object must each have a key value that is distinct from the key values of the other own
properties of that object.
All objects are logically collections of properties, but there are multiple forms of objects that differ in
their semantics for accessing and manipulating their properties. Please see 6.1.7.2
for definitions of the multiple forms of objects.
In addition, some objects are callable; these are referred to as functions or function
objects and are described further below. All functions in ECMAScript are members of the
Object type.
6.1.7.1 Property Attributes
Attributes are used in this specification to define and explain the state of Object properties as
described in Table 3. Unless
specified explicitly, the initial value of each attribute is its Default Value.
If the value is an Object it must be
a function object. The
function's [[Call]] internal method (Table
5) is called with an empty arguments list to retrieve the property value each
time a get access of the property is performed.
If the value is an Object it must be
a function object. The
function's [[Call]] internal method (Table
5) is called with an arguments list containing the assigned value as its sole
argument each time a set access of the property is performed. The effect of a property's [[Set]] internal method may, but is not required to, have an effect on the
value returned by subsequent calls to the property's [[Get]] internal
method.
If false, attempts to delete the property, change it from a data property to an
accessor property or
from an accessor property to a
data property, or make
any changes to its attributes (other than replacing an existing [[Value]] or setting [[Writable]] to
false) will fail.
6.1.7.2 Object Internal Methods and Internal Slots
The actual semantics of objects, in ECMAScript, are specified via algorithms called internal
methods. Each object in an ECMAScript engine is associated with a set of internal methods that
defines its runtime behaviour. These internal methods are not part of the ECMAScript language. They are
defined by this specification purely for expository purposes. However, each object within an
implementation of ECMAScript must behave as specified by the internal methods associated with it. The
exact manner in which this is accomplished is determined by the implementation.
Internal method names are polymorphic. This means that different object values may perform different
algorithms when a common internal method name is invoked upon them. That actual object upon which an
internal method is invoked is the “target” of the invocation. If, at runtime, the implementation of an
algorithm attempts to use an internal method of an object that the object does not support, a
TypeError exception is thrown.
Internal slots correspond to internal state that is associated with objects and used by various
ECMAScript specification algorithms. Internal slots are not object properties and they are not inherited.
Depending upon the specific internal slot specification, such state may consist of values of any ECMAScript language
type or of specific ECMAScript specification type values. Unless explicitly specified
otherwise, internal slots are allocated as part of the process of creating an object and may not be
dynamically added to an object. Unless specified otherwise, the initial value of an internal slot is the
value undefined. Various algorithms within this specification create objects that have
internal slots. However, the ECMAScript language provides no direct way to associate internal slots with
an object.
All objects have an internal slot named [[PrivateElements]], which is a
List of
PrivateElements.
This List
represents the values of the private fields, methods, and accessors for the object. Initially, it is an
empty List.
Internal methods and internal slots are identified within this specification using names enclosed in
double square brackets [[ ]].
Table 4
summarizes the essential internal methods used by this specification that are applicable to all
objects created or manipulated by ECMAScript code. Every object must have algorithms for all of the
essential internal methods. However, all objects do not necessarily use the same algorithms for those
methods.
An ordinary object is an object
that satisfies all of the following criteria:
For the internal methods listed in Table 4, the
object uses those defined in 10.1.
If the object has a [[Call]] internal method, it uses either the one defined in
10.2.1
or the one defined in 10.3.1.
If the object has a [[Construct]] internal method, it uses either the one
defined in 10.2.2
or the one defined in 10.3.2.
An exotic object is an object that
is not an ordinary object.
This specification recognizes different kinds of exotic objects by those objects'
internal methods. An object that is behaviourally equivalent to a particular kind of exotic
object (such as an Array exotic object or a
bound function exotic
object), but does not have the same collection of internal methods specified for that
kind, is not recognized as that kind of exotic object.
The “Signature” column of Table 4 and other
similar tables describes the invocation pattern for each internal method. The invocation pattern always
includes a parenthesized list of descriptive parameter names. If a parameter name is the same as an
ECMAScript type name then the name describes the required type of the parameter value. If an internal
method explicitly returns a value, its parameter list is followed by the symbol “→” and the type name of
the returned value. The type names used in signatures refer to the types defined in clause 6 augmented
by the following additional names. “any” means the value may be any ECMAScript language
type.
In addition to its parameters, an internal method always has access to the object that is the target of
the method invocation.
Determine the object that provides inherited properties for this object. A
null value indicates that there are no inherited properties.
[[SetPrototypeOf]]
(Object | Null) → Boolean
Associate this object with another object that provides inherited properties. Passing
null indicates that there are no inherited properties. Returns
true indicating that the operation was completed successfully or
false indicating that the operation was not successful.
[[IsExtensible]]
( ) → Boolean
Determine whether it is permitted to add additional properties to this object.
[[PreventExtensions]]
( ) → Boolean
Control whether new properties may be added to this object. Returns true if
the operation was successful or false if the operation was unsuccessful.
Return a Property
Descriptor for the own property of this object whose key is
propertyKey, or undefined if no such property exists.
[[DefineOwnProperty]]
(propertyKey, PropertyDescriptor) → Boolean
Create or alter the own property, whose key is propertyKey, to have the state
described by PropertyDescriptor. Return true if that property was
successfully created/updated or false if the property could not be created or
updated.
[[HasProperty]]
(propertyKey) → Boolean
Return a Boolean value indicating whether this object already has either an own or inherited
property whose key is propertyKey.
[[Get]]
(propertyKey, Receiver) →any
Return the value of the property whose key is propertyKey from this object. If any
ECMAScript code must be executed to retrieve the property value, Receiver is used as
the this value when evaluating the code.
[[Set]]
(propertyKey, value, Receiver) → Boolean
Set the value of the property whose key is propertyKey to value. If any
ECMAScript code must be executed to set the property value, Receiver is used as the
this value when evaluating the code. Returns true if the
property value was set or false if it could not be set.
[[Delete]]
(propertyKey) → Boolean
Remove the own property whose key is propertyKey from this object. Return
false if the property was not deleted and is still present. Return
true if the property was deleted or is not present.
Return a List
whose elements are all of the own property keys for the
object.
Table
5 summarizes additional essential internal methods that are supported by objects that
may be called as functions. A function
object is an object that supports the [[Call]] internal method. A constructor is an object that supports the
[[Construct]] internal method. Every object that supports [[Construct]] must support [[Call]]; that is, every
constructor must be a function
object. Therefore, a constructor may also be referred to as
a constructor function or
constructorfunction
object.
Table 5: Additional Essential Internal Methods of Function Objects
Executes code associated with this object. Invoked via a function call expression. The arguments
to the internal method are a this value and a List
whose elements are the arguments passed to the function by a call expression. Objects that
implement this internal method are callable.
Creates an object. Invoked via the new operator or a super call. The
first argument to the internal method is a List
whose elements are the arguments of the constructor invocation or
the super call. The second argument is the object to which the new
operator was initially applied. Objects that implement this internal method are called
constructors. A
function object is not
necessarily a constructor and such
non-constructorfunction objects do not
have a [[Construct]] internal method.
The semantics of the essential internal methods for ordinary objects and standard
exotic
objects are specified in
clause 10. If
any specified use of an internal method of an exotic object is not supported by an
implementation, that usage must throw a TypeError exception when attempted.
6.1.7.3 Invariants of the Essential Internal Methods
The Internal Methods of Objects of an ECMAScript engine must conform to the list of invariants specified
below. Ordinary ECMAScript Objects as well as all standard exotic objects in
this specification maintain these invariants. ECMAScript Proxy objects maintain these invariants by means
of runtime checks on the result of traps invoked on the [[ProxyHandler]] object.
Any implementation provided exotic objects must also maintain
these invariants for those objects. Violation of these invariants may cause ECMAScript code to have
unpredictable behaviour and create security issues. However, violation of these invariants must never
compromise the memory safety of an implementation.
An implementation must not allow these invariants to be circumvented in any manner such as by providing
alternative interfaces that implement the functionality of the essential internal methods without
enforcing their invariants.
Definitions:
The target of an internal method is the object upon which the internal method is called.
A target is non-extensible if it has been observed to return false from its
[[IsExtensible]] internal method, or true from its [[PreventExtensions]] internal method.
A non-existent property is a property that does not exist as an own property on a
non-extensible target.
All references to SameValue are according to
the definition of the SameValue algorithm.
Return value:
The value returned by any internal method must be a Completion
Record with either:
[[Type]] = normal, [[Target]]
= empty, and [[Value]] = a value of the "normal return
type" shown below for that internal method, or
If target is non-extensible, and [[GetPrototypeOf]] returns a value
V, then any future calls to [[GetPrototypeOf]] should return the
SameValue as V.
Note 2
An object's prototype chain should have finite length (that is, starting from
any object, recursively applying the [[GetPrototypeOf]] internal method to
its result should eventually lead to the value null). However, this requirement is
not enforceable as an object level invariant if the prototype chain includes any exotic
objects that do not use the ordinary object definition of
[[GetPrototypeOf]]. Such a circular prototype chain may result in infinite
loops when accessing object properties.
[[SetPrototypeOf]] ( V )
The normal return type is Boolean.
If target is non-extensible, [[SetPrototypeOf]] must return
false, unless V is the SameValue as the target's observed
[[GetPrototypeOf]] value.
[[IsExtensible]] ( )
The normal return type is Boolean.
If [[IsExtensible]] returns false, all future calls to [[IsExtensible]] on the target must return false.
[[PreventExtensions]] ( )
The normal return type is Boolean.
If [[PreventExtensions]] returns true, all future calls to
[[IsExtensible]] on the target must return false and the
target is now considered non-extensible.
If P is described as a non-configurable, non-writable own data property,
all future calls to [[GetOwnProperty]] ( P ) must return Property
Descriptor whose [[Value]] is SameValue as
P's [[Value]] attribute.
If P's attributes other than [[Writable]] and [[Value]] may change over time, or if the property might be deleted, then
P's [[Configurable]] attribute must be true.
If the [[Writable]] attribute may change from false to
true, then the [[Configurable]] attribute must be
true.
If the target is non-extensible and P is non-existent, then all future calls to [[GetOwnProperty]] (P) on the target must describe P as
non-existent (i.e. [[GetOwnProperty]] (P) must return
undefined).
Note 3
As a consequence of the third invariant, if a property is described as a data
property and it may return different values over time, then either or both of the
[[Writable]] and [[Configurable]] attributes must be
true even if no mechanism to change the value is exposed via the other essential
internal methods.
[[DefineOwnProperty]] ( P, Desc )
The normal return type is Boolean.
[[DefineOwnProperty]] must return false if P has
previously been observed as a non-configurable own property of the target, unless either:
All attributes of Desc are the SameValue as P's
attributes.
[[DefineOwnProperty]] (P, Desc) must return
false if target is non-extensible and P is a non-existent own property.
That is, a non-extensible target object cannot be extended with new properties.
[[HasProperty]] ( P )
The normal return type is Boolean.
If P was previously observed as a non-configurable own data or accessor
property of the target,
[[HasProperty]] must return true.
If P was previously observed as a non-configurable, non-writable own data
property of the target with value V, then [[Get]]
must return the SameValue as V.
If P was previously observed as a non-configurable own accessor
property of the target whose [[Get]] attribute is
undefined, the [[Get]] operation must return
undefined.
[[Set]] ( P, V, Receiver )
The normal return type is Boolean.
If P was previously observed as a non-configurable, non-writable own data
property of the target, then [[Set]] must return
false unless V is the SameValue as P's [[Value]] attribute.
If P was previously observed as a non-configurable own accessor
property of the target whose [[Set]] attribute is
undefined, the [[Set]] operation must return
false.
[[Delete]] ( P )
The normal return type is Boolean.
If P was previously observed as a non-configurable own data or accessor
property of the target,
[[Delete]] must return false.
The returned List must
not contain any duplicate entries.
The Type of each element of the returned List is
either String or Symbol.
The returned List must
contain at least the keys of all non-configurable own properties that have previously been observed.
If the target is non-extensible, the returned List must
contain only the keys of all own properties of the target that are observable using [[GetOwnProperty]].
The target must also have a [[Call]] internal method.
6.1.7.4 Well-Known Intrinsic Objects
Well-known intrinsics are built-in objects that are explicitly referenced by the algorithms of this
specification and which usually have realm-specific identities. Unless otherwise
specified each intrinsic object actually corresponds to a set of similar objects, one per realm.
Within this specification a reference such as %name% means the intrinsic object, associated with the
current realm, corresponding to the name. A
reference such as %name.a.b% means, as if the "b" property of the value of the
"a" property of the intrinsic object %name% was accessed prior to any ECMAScript code
being evaluated. Determination of the current realm and its intrinsics is described in
9.4. The well-known
intrinsics are listed in Table 6.
A specification type corresponds to meta-values that are used within algorithms to describe the semantics of
ECMAScript language constructs and ECMAScript language
types. The specification types include Reference, List, Completion
Record, Property
Descriptor, Environment Record, Abstract Closure, and Data
Block. Specification type values are specification artefacts that do not necessarily
correspond to any specific entity within an ECMAScript implementation. Specification type values may be used
to describe intermediate results of ECMAScript expression evaluation but such values cannot be stored as
properties of objects or values of ECMAScript language variables.
6.2.1 The Enum Specification Type
Enums are values which are internal to the specification and
not directly observable from ECMAScript code. Enums are denoted using a sans-serif
typeface. For instance, a Completion
Record's [[Type]] field takes on values like
normal, return, or throw. Enums have no
characteristics other than their name. The name of an enum serves no purpose other than to distinguish it
from other enums, and implies nothing about its usage or meaning in context.
6.2.2 The List and Record Specification Types
The List type is used to explain the evaluation of argument lists
(see 13.3.8) in new
expressions, in function calls, and in other algorithms where a simple ordered list of values is needed.
Values of the List type are simply ordered sequences of list elements containing the individual values.
These sequences may be of any length. The elements of a list may be randomly accessed using 0-origin
indices. For notational convenience an array-like syntax can be used to access List elements. For example,
arguments[2] is shorthand for saying the 3rd element of the List arguments.
When an algorithm iterates over the elements of a List without specifying an order, the order used is the
order of the elements in the List.
For notational convenience within this specification, a literal syntax can be used to express a new List
value. For example, « 1, 2 » defines a List value that has two elements each of which is initialized to a
specific value. A new empty List can be expressed as « ».
In this specification, the phrase "the list-concatenation
of A, B, ..." (where each argument is a possibly empty List) denotes a new List value
whose elements are the concatenation of the elements (in order) of each of the arguments (in order).
The Record type is used to describe data aggregations within
the algorithms of this specification. A Record type value consists of one or more named fields. The value of
each field is an ECMAScript language
value or specification value. Field names are always enclosed in double brackets, for
example [[Value]].
For notational convenience within this specification, an object literal-like syntax can be used to express
a Record value. For example, { [[Field1]]: 42, [[Field2]]:
false, [[Field3]]: empty } defines a
Record value that has three fields, each of which is initialized to a specific value. Field name order is
not significant. Any fields that are not explicitly listed are considered to be absent.
In specification text and algorithms, dot notation may be used to refer to a specific field of a Record
value. For example, if R is the record shown in the previous paragraph then R.[[Field2]] is shorthand for “the field of R named [[Field2]]”.
Schema for commonly used Record field combinations may be named, and that name may be used as a prefix to a
literal Record value to identify the specific kind of aggregations that is being described. For example:
PropertyDescriptor { [[Value]]: 42, [[Writable]]:
false, [[Configurable]]: true }.
6.2.3 The Set and Relation Specification Types
The Set type is used to explain a collection of unordered elements for use in the memory
model. It is distinct from the ECMAScript collection type of the same name. To
disambiguate, instances of the ECMAScript collection are consistently referred to as "Set objects" within
this specification. Values of the Set type are simple collections of elements, where no element appears more
than once. Elements may be added to and removed from Sets. Sets may be unioned, intersected, or subtracted
from each other.
The Relation type is used to explain constraints on Sets.
Values of the Relation type are Sets of ordered pairs of values from its value domain. For example, a
Relation on events is a set of ordered pairs of events. For a Relation R and two values
a and b in the value domain of R, aRb is
shorthand for saying the ordered pair (a, b) is a member of R. A Relation
is least with respect to some conditions when it is the smallest Relation that satisfies those conditions.
A strict partial order is a Relation value
R that satisfies the following.
For all a, b, and c in R's domain:
It is not the case that aRa, and
If aRb and bRc, then
aRc.
Note 1
The two properties above are called irreflexivity and transitivity, respectively.
A strict total order is a Relation value
R that satisfies the following.
For all a, b, and c in R's domain:
a is b or aRb or bRa, and
It is not the case that aRa, and
If aRb and bRc, then
aRc.
Note 2
The three properties above are called totality, irreflexivity, and transitivity, respectively.
6.2.4 The Completion Record Specification Type
The Completion Record specification type is used to
explain the runtime propagation of values and control flow such as the behaviour of statements
(break, continue, return and throw) that perform
nonlocal transfers of control.
Completion Records have the fields defined in Table
7.
The following shorthand terms are sometimes used to refer to Completion Records.
normal completion refers to any Completion Record
with a [[Type]] value of normal.
break completion refers to any Completion Record
with a [[Type]] value of break.
continue completion refers to any Completion
Record with a [[Type]] value of continue.
return completion refers to any Completion Record
with a [[Type]] value of return.
throw completion refers to any Completion Record
with a [[Type]] value of throw.
abrupt completion refers to any Completion Record
with a [[Type]] value other than normal.
a normal completion containing some
type of value refers to a normal completion that has a value of that type in its [[Value]] field.
Callable objects that are defined in this specification only return a normal completion or a throw
completion. Returning any other kind of Completion Record is considered an editorial error.
Implementation-defined
callable objects must return either a normal completion or a throw completion.
6.2.4.1 NormalCompletion ( value )
The abstract operation NormalCompletion takes argument value (any value except a Completion
Record) and returns a normal
completion. It performs the following steps when called:
The abstract operation ThrowCompletion takes argument value (an ECMAScript language
value) and returns a throw
completion. It performs the following steps when called:
The abstract operation UpdateEmpty takes arguments completionRecord (a Completion
Record) and value (any value except a Completion
Record) and returns a Completion
Record. It performs the following steps when called:
The Reference Record type is used to explain the
behaviour of such operators as delete, typeof, the assignment operators, the
superkeyword and other
language features. For example, the left-hand operand of an assignment is expected to produce a Reference
Record.
A Reference Record is a resolved name or property binding; its fields are defined by Table 8.
If not empty, the Reference
Record represents a property binding that was expressed using the
superkeyword;
it is called a Super
Reference Record and its [[Base]] value will never be an
Environment
Record. In that case, the [[ThisValue]] field holds
the this value at the time the Reference
Record was created.
The following abstract
operations are used in this specification to operate upon Reference Records:
6.2.5.1 IsPropertyReference ( V )
The abstract operation IsPropertyReference takes argument V (a Reference
Record) and returns a Boolean. It performs the following steps when called:
1. If V.[[Base]] is
unresolvable, return false.
2. If V.[[Base]] is an Environment Record,
return false; otherwise return true.
6.2.5.2 IsUnresolvableReference ( V )
The abstract operation IsUnresolvableReference takes argument V (a Reference
Record) and returns a Boolean. It performs the following steps when called:
1. If V.[[Base]] is
unresolvable, return true; otherwise return
false.
6.2.5.3 IsSuperReference ( V )
The abstract operation IsSuperReference takes argument V (a Reference
Record) and returns a Boolean. It performs the following steps when called:
1. If V.[[ThisValue]] is not
empty, return true; otherwise return
false.
6.2.5.4 IsPrivateReference ( V )
The abstract operation IsPrivateReference takes argument V (a Reference
Record) and returns a Boolean. It performs the following steps when called:
1. If V.[[ReferencedName]] is a
Private Name, return
true; otherwise return false.
c. Return ? base.GetBindingValue(V.[[ReferencedName]], V.[[Strict]]) (see
9.1).
Note
The object that may be created in step 3.a is not accessible
outside of the above abstract operation and the ordinary object[[Get]] internal method. An implementation might choose to avoid the actual
creation of the object.
c. Return ? base.SetMutableBinding(V.[[ReferencedName]], W, V.[[Strict]]) (see 9.1).
Note
The object that may be created in step 3.a is not accessible
outside of the above abstract operation and the ordinary object[[Set]] internal method. An implementation might choose to avoid the actual
creation of that object.
The abstract operation MakePrivateReference takes arguments baseValue (an ECMAScript language
value) and privateIdentifier (a String) and returns a Reference
Record. It performs the following steps when called:
4. Return the Reference
Record { [[Base]]: baseValue, [[ReferencedName]]: privateName, [[Strict]]:
true, [[ThisValue]]: empty }.
6.2.6 The Property Descriptor Specification Type
The Property Descriptor type is used to explain
the manipulation and reification of Object property attributes. A Property Descriptor is a Record with
zero or more fields, where each field's name is an attribute name and its value is a corresponding attribute
value as specified in 6.1.7.1. The schema name
used within this specification to tag literal descriptions of Property Descriptor records is
“PropertyDescriptor”.
Property Descriptor values may be further classified as data Property Descriptors and accessor Property
Descriptors based upon the existence or use of certain fields. A data Property Descriptor is one that
includes any fields named either [[Value]] or [[Writable]]. An accessor Property Descriptor is one that includes any fields named
either [[Get]] or [[Set]]. Any Property Descriptor may
have fields named [[Enumerable]] and [[Configurable]]. A
Property Descriptor value may not be both a data Property Descriptor and an accessor Property Descriptor;
however, it may be neither (in which case it is a generic Property Descriptor). A fully
populated Property Descriptor is one that is either an accessor Property Descriptor or a data
Property Descriptor and that has all of the corresponding fields defined in Table 3.
The following abstract
operations are used in this specification to operate upon Property Descriptor values:
6.2.6.1 IsAccessorDescriptor ( Desc )
The abstract operation IsAccessorDescriptor takes argument Desc (a Property
Descriptor or undefined) and returns a Boolean. It performs the
following steps when called:
1. If Desc is undefined, return
false.
2. If Desc has a [[Get]] field,
return true.
3. If Desc has a [[Set]] field,
return true.
4. Return false.
6.2.6.2 IsDataDescriptor ( Desc )
The abstract operation IsDataDescriptor takes argument Desc (a Property
Descriptor or undefined) and returns a Boolean. It performs the
following steps when called:
1. If Desc is undefined, return
false.
2. If Desc has a [[Value]]
field, return true.
3. If Desc has a [[Writable]]
field, return true.
4. Return false.
6.2.6.3 IsGenericDescriptor ( Desc )
The abstract operation IsGenericDescriptor takes argument Desc (a Property
Descriptor or undefined) and returns a Boolean. It performs the
following steps when called:
The abstract operation FromPropertyDescriptor takes argument Desc (a Property
Descriptor or undefined) and returns an Object or
undefined. It performs the following steps when called:
b. If IsCallable(setter)
is false and setter is not undefined, throw a
TypeError exception.
c. Set desc.[[Set]] to
setter.
15. If desc has a [[Get]] field
or desc has a [[Set]] field, then
a. If desc has a [[Value]]
field or desc has a [[Writable]] field, throw a
TypeError exception.
16. Return desc.
6.2.6.6 CompletePropertyDescriptor ( Desc )
The abstract operation CompletePropertyDescriptor takes argument Desc (a Property
Descriptor) and returns unused. It performs the following steps
when called:
1. Let like be the Record
{ [[Value]]: undefined, [[Writable]]: false, [[Get]]:
undefined, [[Set]]: undefined, [[Enumerable]]: false, [[Configurable]]: false }.
a. If Desc does not have a [[Value]] field, set Desc.[[Value]] to
like.[[Value]].
b. If Desc does not have a [[Writable]] field, set Desc.[[Writable]] to like.[[Writable]].
3. Else,
a. If Desc does not have a [[Get]] field, set Desc.[[Get]] to
like.[[Get]].
b. If Desc does not have a [[Set]] field, set Desc.[[Set]] to
like.[[Set]].
4. If Desc does not have an [[Enumerable]] field, set Desc.[[Enumerable]] to like.[[Enumerable]].
5. If Desc does not have a [[Configurable]] field, set Desc.[[Configurable]] to like.[[Configurable]].
6. Return unused.
6.2.7 The Environment Record Specification Type
The Environment Record type is
used to explain the behaviour of name resolution in nested functions and blocks. This type and the
operations upon it are defined in 9.1.
6.2.8 The Abstract Closure Specification Type
The Abstract Closure specification type is used to
refer to algorithm steps together with a collection of values. Abstract Closures are meta-values and are
invoked using function application style such as closure(arg1, arg2). Like
abstract
operations, invocations perform the algorithm steps described by the Abstract Closure.
In algorithm steps that create an Abstract Closure, values are captured with the verb "capture" followed by
a list of aliases. When an Abstract Closure is created, it captures the value that is associated with each
alias at that time. In steps that specify the algorithm to be performed when an Abstract Closure is called,
each captured value is referred to by the alias that was used to capture the value.
The Data Block specification type is used to describe a
distinct and mutable sequence of byte-sized (8 bit) numeric values. A byte value is an integer in the inclusive
interval from 0 to 255.
A Data Block value is created with a fixed number of bytes that each have the initial value 0.
For notational convenience within this specification, an array-like syntax can be used to access the
individual bytes of a Data Block value. This notation presents a Data Block value as a 0-origined
integer-indexed sequence of bytes. For example, if db is a 5 byte Data
Block value then db[2] can be used to access its 3rd byte.
A data block that resides in memory that can be referenced from multiple agents concurrently is
designated a Shared Data Block. A Shared Data Block
has an identity (for the purposes of equality testing Shared Data Block values) that is
address-free: it is tied not to the virtual addresses the block is mapped to in any process, but to
the set of locations in memory that the block represents. Two data blocks are equal only if the sets of the
locations they contain are equal; otherwise, they are not equal and the intersection of the sets of
locations they contain is empty. Finally, Shared Data Blocks can be distinguished from Data Blocks.
1. Let db be a new Shared Data
Block value consisting of size bytes. If it is impossible to create such
a Shared Data Block, throw a
RangeError exception.
a. Append WriteSharedMemory
{ [[Order]]: init, [[NoTear]]: true, [[Block]]:
db, [[ByteIndex]]: i, [[ElementSize]]: 1, [[Payload]]:
zero } to eventsRecord.[[EventList]].
The abstract operation CopyDataBlockBytes takes arguments toBlock (a Data
Block or a Shared Data Block),
toIndex (a non-negative integer), fromBlock (a
Data
Block or a Shared
Data Block), fromIndex (a non-negative integer), and
count (a non-negative integer) and returns
unused. It performs the following steps when called:
1. Assert: fromBlock and
toBlock are distinct values.
2. Let fromSize be the number of bytes in
fromBlock.
iii. Let bytes be a List
whose sole element is a nondeterministically chosen byte
value.
iv. NOTE: In implementations, bytes is the result
of a non-atomic read instruction on the underlying hardware. The nondeterminism is a semantic
prescription of the memory model to
describe observable behaviour of hardware with weak consistency.
v. Let readEvent be ReadSharedMemory
{ [[Order]]: unordered, [[NoTear]]: true, [[Block]]:
fromBlock, [[ByteIndex]]: fromIndex, [[ElementSize]]: 1 }.
vi. Append readEvent to
eventsRecord.[[EventList]].
vii. Append Chosen Value
Record { [[Event]]: readEvent, [[ChosenValue]]: bytes } to execution.[[ChosenValues]].
The PrivateElement type is a Record used
in the specification of private class fields, methods, and accessors. Although Property
Descriptors are not used for private elements, private fields behave similarly to
non-configurable, non-enumerable, writable data properties, private methods
behave similarly to non-configurable, non-enumerable, non-writable data properties, and
private accessors behave similarly to non-configurable, non-enumerable accessor properties.
Values of the PrivateElement type are Record values
whose fields are defined by Table 9. Such values are
referred to as PrivateElements.
6.2.11 The ClassFieldDefinition Record Specification Type
The ClassFieldDefinition type is a Record used
in the specification of class fields.
Values of the ClassFieldDefinition type are Record values
whose fields are defined by Table 10. Such
values are referred to as ClassFieldDefinition
Records.
The Private Name specification type is used to describe a
globally unique value (one which differs from any other Private Name, even if they are otherwise
indistinguishable) which represents the key of a private class element (field, method, or accessor). Each
Private Name has an associated immutable [[Description]] which is a
String value. A Private Name may be installed on any ECMAScript object with PrivateFieldAdd or PrivateMethodOrAccessorAdd,
and then read or written using PrivateGet and PrivateSet.
6.2.13 The ClassStaticBlockDefinition Record Specification Type
A ClassStaticBlockDefinition Record
is a Record value
used to encapsulate the executable code for a class static initialization block.
ClassStaticBlockDefinition Records have the fields listed in Table
11.
The function object to be
called during static initialization of a class.
7 Abstract Operations
These operations are not a part of the ECMAScript language; they are defined here solely to aid the
specification of the semantics of the ECMAScript language. Other, more specialized abstract
operations are defined throughout this specification.
7.1 Type Conversion
The ECMAScript language implicitly performs automatic type conversion as needed. To clarify the semantics of
certain constructs it is useful to define a set of conversion abstract
operations. The conversion abstract
operations are polymorphic; they can accept a value of any ECMAScript language
type. But no other specification types are used with these operations.
The BigInt type
has no implicit conversions in the ECMAScript language; programmers must call BigInt explicitly to convert
values from other types.
7.1.1 ToPrimitive ( input [ , preferredType ] )
The abstract operation ToPrimitive takes argument input (an ECMAScript language
value) and optional argument preferredType (string or
number) and returns either a normal completion
containing an ECMAScript language
value or a throw
completion. It converts its input argument to a non-Object
type. If an object is capable of converting to more than one primitive type, it may use
the optional hint preferredType to favour that type. It performs the following steps when called:
When ToPrimitive is called without a hint, then it generally behaves as if the hint were
number. However, objects may over-ride this behaviour by defining a @@toPrimitive method. Of
the objects defined in this specification only Dates (see 21.4.4.45) and
Symbol objects (see 20.4.3.5)
over-ride the default ToPrimitive behaviour. Dates treat the absence of a hint as if the hint were
string.
The abstract operation ToBoolean takes argument argument (an ECMAScript language
value) and returns a Boolean. It converts argument to a value of type Boolean.
It performs the following steps when called:
2. If argument is one of undefined,
null, +0𝔽, -0𝔽,
NaN, 0ℤ, or the empty String, return
false.
3. NOTE: This step is
replaced in section B.3.6.1.
4. Return true.
7.1.3 ToNumeric ( value )
The abstract operation ToNumeric takes argument value (an ECMAScript language
value) and returns either a normal
completion containing either a Number or a BigInt, or a throw
completion. It returns value converted to a Number or a BigInt. It performs
the following steps when called:
The abstract operation RoundMVResult takes argument n (a mathematical value) and
returns a Number. It converts n to a Number in an implementation-defined
manner. For the purposes of this abstract operation, a digit is significant if it is not zero or there
is a non-zero digit to its left and there is a non-zero digit to its right. For the purposes of this
abstract operation, "the mathematical value denoted
by" a representation of a mathematical value is the
inverse of "the decimal representation of" a mathematical value. It
performs the following steps when called:
1. If the decimal representation of n has 20 or fewer
significant digits, return 𝔽(n).
2. Let option1 be the mathematical value
denoted by the result of replacing each significant digit in the decimal representation of
n after the 20th with a 0 digit.
3. Let option2 be the mathematical value
denoted by the result of replacing each significant digit in the decimal representation of
n after the 20th with a 0 digit and then incrementing it at the 20th position (with
carrying as necessary).
The abstract operation ToIntegerOrInfinity takes argument argument (an ECMAScript language
value) and returns either a normal
completion containing either an integer, +∞, or -∞, or a throw
completion. It converts argument to an integer representing its
Number value with fractional part truncated, or to +∞ or -∞ when that Number value is infinite. It performs
the following steps when called:
𝔽(ToIntegerOrInfinity(x))
never returns -0𝔽 for any value of x. The truncation of the
fractional part is performed after converting x to a mathematical
value.
Step 5 is the only difference
between ToUint32 and ToInt32.
The ToUint32 abstract operation is idempotent: if applied to a result that it produced, the second
application leaves that value unchanged.
ToUint32(ToInt32(x)) is the same
value as ToUint32(x) for all values of x. (It is to preserve this latter
property that +∞𝔽 and -∞𝔽 are mapped to
+0𝔽.)
8. If f is even, return 𝔽(f).
Otherwise, return 𝔽(f + 1).
Note
Unlike most other ECMAScript integer conversion operations,
ToUint8Clamp rounds rather than truncates non-integral values. It also uses “round half to even”
tie-breaking, which differs from the “round half up” tie-breaking of Math.round.
7.1.13 ToBigInt ( argument )
The abstract operation ToBigInt takes argument argument (an ECMAScript language
value) and returns either a normal
completion containing a BigInt or a throw
completion. It converts argument to a BigInt value, or throws if an implicit
conversion from Number would be required. It performs the following steps when called:
The abstract operation CanonicalNumericIndexString takes argument argument (a String) and
returns a Number or undefined. If argument is either "-0"
or exactly matches the result of ToString(n) for some Number
value n, it returns the respective Number value. Otherwise, it returns
undefined. It performs the following steps when called:
The abstract operation IsCallable takes argument argument (an ECMAScript language
value) and returns a Boolean. It determines if argument is a callable function
with a [[Call]] internal method. It performs the following steps when called:
2. If argument has a [[Call]]
internal method, return true.
3. Return false.
7.2.4 IsConstructor ( argument )
The abstract operation IsConstructor takes argument argument (an ECMAScript language
value) and returns a Boolean. It determines if argument is a function
object with a [[Construct]] internal method. It performs the
following steps when called:
2. If argument has a [[Construct]]
internal method, return true.
3. Return false.
7.2.5 IsExtensible ( O )
The abstract operation IsExtensible takes argument O (an Object) and returns either a normal completion
containing a Boolean or a throw
completion. It is used to determine whether additional properties can be added to
O. It performs the following steps when called:
1. Return ? O.[[IsExtensible]]().
7.2.6 IsIntegralNumber ( argument )
The abstract operation IsIntegralNumber takes argument argument (an ECMAScript language
value) and returns a Boolean. It determines if argument is a finiteintegral Number value. It performs
the following steps when called:
3. If truncate(ℝ(argument)) ≠
ℝ(argument), return
false.
4. Return true.
7.2.7 IsPropertyKey ( argument )
The abstract operation IsPropertyKey takes argument argument (an ECMAScript language
value) and returns a Boolean. It determines if argument is a value that may be
used as a property key. It performs the
following steps when called:
The abstract operation IsStringWellFormedUnicode takes argument string (a String) and returns a
Boolean. It interprets string as a sequence of UTF-16 encoded code points, as described in
6.1.4, and
determines whether it is a well
formed UTF-16 sequence. It performs the following steps when called:
b. If cp.[[IsUnpairedSurrogate]] is true, return
false.
c. Set k to k + cp.[[CodeUnitCount]].
4. Return true.
7.2.10 SameValue ( x, y )
The abstract operation SameValue takes arguments x (an ECMAScript language
value) and y (an ECMAScript language
value) and returns a Boolean. It determines whether or not the two arguments are the same
value. It performs the following steps when called:
This algorithm differs from the IsStrictlyEqual Algorithm by
treating all NaN values as equivalent and by differentiating
+0𝔽 from -0𝔽.
7.2.11 SameValueZero ( x, y )
The abstract operation SameValueZero takes arguments x (an ECMAScript language
value) and y (an ECMAScript language
value) and returns a Boolean. It determines whether or not the two arguments are the same
value (ignoring the difference between +0𝔽 and
-0𝔽). It performs the following steps when called:
SameValueZero differs from SameValue only in that it treats
+0𝔽 and -0𝔽 as equivalent.
7.2.12 SameValueNonNumber ( x, y )
The abstract operation SameValueNonNumber takes arguments x (an ECMAScript language
value, but not a Number) and y (an ECMAScript language
value, but not a Number) and returns a Boolean. It performs the following steps when
called:
7. If x is y, return true;
otherwise, return false.
Note 1
For expository purposes, some cases are handled separately within this algorithm even if it is unnecessary
to do so.
Note 2
The specifics of what "x is y" means are detailed in 5.2.7.
7.2.13 IsLessThan ( x, y, LeftFirst )
The abstract operation IsLessThan takes arguments x (an ECMAScript language
value), y (an ECMAScript language
value), and LeftFirst (a Boolean) and returns either a normal completion
containing either a Boolean or undefined, or a throw
completion. It provides the semantics for the comparison x < y,
returning true, false, or undefined (which
indicates that at least one operand is NaN). The LeftFirst flag is used to
control the order in which operations with potentially visible side-effects are performed upon x
and y. It is necessary because ECMAScript specifies left to right evaluation of expressions. If
LeftFirst is true, the x parameter corresponds to an expression
that occurs to the left of the y parameter's corresponding expression. If LeftFirst is
false, the reverse is the case and operations must be performed upon y before
x. It performs the following steps when called:
k. If ℝ(nx) < ℝ(ny), return
true; otherwise return false.
Note 1
Step 3 differs from step
1.c in the algorithm
that handles the addition operator + (13.15.3)
by using the logical-and operation instead of the logical-or operation.
Note 2
The comparison of Strings uses a simple lexicographic ordering on sequences of UTF-16 code unit values.
There is no attempt to use the more complex, semantically oriented definitions of character or string
equality and collating order defined in the Unicode specification. Therefore String values that are
canonically equal according to the Unicode Standard but not in the same normalization form could test as
unequal. Also note that lexicographic ordering by code unit differs from ordering by code
point for Strings containing surrogate pairs.
a. If x is not finite or
y is not finite, return
false.
b. If ℝ(x) = ℝ(y),
return
true; otherwise return false.
14. Return false.
7.2.15 IsStrictlyEqual ( x, y )
The abstract operation IsStrictlyEqual takes arguments x (an ECMAScript language
value) and y (an ECMAScript language
value) and returns a Boolean. It provides the semantics for the ===
operator. It performs the following steps when called:
This algorithm differs from the SameValue Algorithm in its
treatment of signed zeroes and NaNs.
7.3 Operations on Objects
7.3.1 MakeBasicObject ( internalSlotsList )
The abstract operation MakeBasicObject takes argument internalSlotsList (a List of
internal slot names) and returns an Object. It is the source of all ECMAScript objects that are created
algorithmically, including both ordinary objects and exotic
objects. It factors out common steps used in creating all objects, and centralizes object
creation. It performs the following steps when called:
1. Let obj be a newly created object with an internal slot
for each name in internalSlotsList.
2. Set obj's essential internal methods to the default
ordinary object definitions
specified in 10.1.
3. Assert: If the caller will not be
overriding both obj's [[GetPrototypeOf]] and [[SetPrototypeOf]] essential internal methods, then internalSlotsList
contains [[Prototype]].
4. Assert: If the caller will not be
overriding all of obj's [[SetPrototypeOf]], [[IsExtensible]], and [[PreventExtensions]] essential
internal methods, then internalSlotsList contains [[Extensible]].
5. If internalSlotsList contains [[Extensible]], set obj.[[Extensible]] to
true.
6. Return obj.
Note
Within this specification, exotic objects are created in
abstract
operations such as ArrayCreate and BoundFunctionCreate by
first calling MakeBasicObject to obtain a basic, foundational object, and then overriding some or all of
that object's internal methods. In order to encapsulate exotic object
creation, the object's essential internal methods are never modified outside those operations.
The abstract operation Set takes arguments O (an Object), P (a property
key), V (an ECMAScript language
value), and Throw (a Boolean) and returns either a normal completion
containingunused or a throw
completion. It is used to set the value of a specific property of an object. V
is the new value for the property. It performs the following steps when called:
1. Let success be ? O.[[Set]](P, V,
O).
2. If success is false and
Throw is true, throw a TypeError exception.
1. Let newDesc be the PropertyDescriptor { [[Value]]: V, [[Writable]]:
true, [[Enumerable]]: true, [[Configurable]]: true }.
2. Return ? O.[[DefineOwnProperty]](P, newDesc).
Note
This abstract operation creates a property whose attributes are set to the same defaults used for
properties created by the ECMAScript language assignment operator. Normally, the property will not
already exist. If it does exist and is not configurable or if O is not extensible, [[DefineOwnProperty]] will return false.
7.3.6 CreateDataPropertyOrThrow ( O, P, V )
The abstract operation CreateDataPropertyOrThrow takes arguments O (an Object), P (a
property key), and V (an
ECMAScript language
value) and returns either a normal
completion containingunused or a throw
completion. It is used to create a new own property of an object. It throws a
TypeError exception if the requested property update cannot be performed. It performs the
following steps when called:
2. If success is false, throw a
TypeError exception.
3. Return unused.
Note
This abstract operation creates a property whose attributes are set to the same defaults used for
properties created by the ECMAScript language assignment operator. Normally, the property will not
already exist. If it does exist and is not configurable or if O is not extensible, [[DefineOwnProperty]] will return false causing this operation
to throw a TypeError exception.
7.3.7 CreateNonEnumerableDataPropertyOrThrow ( O, P,
V )
The abstract operation CreateNonEnumerableDataPropertyOrThrow takes arguments O (an Object),
P (a property key), and V (an
ECMAScript language
value) and returns unused. It is used to create a new
non-enumerable own property of an ordinary object. It performs the
following steps when called:
1. Assert: O is an ordinary,
extensible object with no non-configurable properties.
2. Let newDesc be the PropertyDescriptor { [[Value]]: V, [[Writable]]:
true, [[Enumerable]]: false, [[Configurable]]: true }.
This abstract operation creates a property whose attributes are set to the same defaults used for
properties created by the ECMAScript language assignment operator except it is not enumerable. Normally,
the property will not already exist. If it does exist, DefinePropertyOrThrow
is guaranteed to complete normally.
7.3.8 DefinePropertyOrThrow ( O, P, desc )
The abstract operation DefinePropertyOrThrow takes arguments O (an Object), P (a
property key), and desc
(a Property
Descriptor) and returns either a normal completion
containingunused or a throw
completion. It is used to call the [[DefineOwnProperty]]
internal method of an object in a manner that will throw a TypeError exception if the
requested property update cannot be performed. It performs the following steps when called:
1. Let success be ? O.[[DefineOwnProperty]](P,
desc).
2. If success is false, throw a
TypeError exception.
3. Return unused.
7.3.9 DeletePropertyOrThrow ( O, P )
The abstract operation DeletePropertyOrThrow takes arguments O (an Object) and P (a
property key) and returns either a
normal completion
containingunused or a throw
completion. It is used to remove a specific own property of an object. It throws an
exception if the property is not configurable. It performs the following steps when called:
1. Let success be ? O.[[Delete]](P).
2. If success is false, throw a
TypeError exception.
2. If func is either undefined or
null, return undefined.
3. If IsCallable(func) is
false, throw a TypeError exception.
4. Return func.
7.3.11 HasProperty ( O, P )
The abstract operation HasProperty takes arguments O (an Object) and P (a property
key) and returns either a normal
completion containing a Boolean or a throw
completion. It is used to determine whether an object has a property with the specified
property key. The property may be
either own or inherited. It performs the following steps when called:
1. Return ? O.[[HasProperty]](P).
7.3.12 HasOwnProperty ( O, P )
The abstract operation HasOwnProperty takes arguments O (an Object) and P (a
property key) and returns either a
normal completion
containing a Boolean or a throw
completion. It is used to determine whether an object has an own property with the
specified property key. It performs the
following steps when called:
The abstract operation Construct takes argument F (a constructor) and
optional arguments argumentsList (a List of
ECMAScript language
values) and newTarget (a constructor) and returns either a
normal completion
containing an Object or a throw
completion. It is used to call the [[Construct]] internal method
of a function object.
argumentsList and newTarget are the values to be passed as the corresponding arguments
of the internal method. If argumentsList is not present, a new empty List is used
as its value. If newTarget is not present, F is used as its value. It performs the
following steps when called:
1. If newTarget is not present, set newTarget to
F.
2. If argumentsList is not present, set
argumentsList to a new empty List.
If newTarget is not present, this operation is equivalent to:
new F(...argumentsList)
7.3.15 SetIntegrityLevel ( O, level )
The abstract operation SetIntegrityLevel takes arguments O (an Object) and level
(sealed or frozen) and returns either a normal completion
containing a Boolean or a throw
completion. It is used to fix the set of own properties of an object. It performs the
following steps when called:
1. Let status be ? O.[[PreventExtensions]]().
2. If status is false, return
false.
3. Let keys be ? O.[[OwnPropertyKeys]]().
4. If level is sealed, then
a. For each element k of keys, do
i. Perform ? DefinePropertyOrThrow(O, k,
PropertyDescriptor { [[Configurable]]:
false }).
The abstract operation TestIntegrityLevel takes arguments O (an Object) and level
(sealed or frozen) and returns either a normal completion
containing a Boolean or a throw
completion. It is used to determine if the set of own properties of an object are fixed.
It performs the following steps when called:
3. NOTE: If the object is extensible, none of its properties are
examined.
4. Let keys be ? O.[[OwnPropertyKeys]]().
5. For each element k of keys, do
a. Let currentDesc be ? O.[[GetOwnProperty]](k).
b. If currentDesc is not undefined,
then
i. If currentDesc.[[Configurable]] is true, return
false.
ii. If level is frozen and
IsDataDescriptor(currentDesc)
is true, then
1. If currentDesc.[[Writable]] is true, return
false.
6. Return true.
7.3.17 CreateArrayFromList ( elements )
The abstract operation CreateArrayFromList takes argument elements (a List of
ECMAScript language
values) and returns an Array. It is used to create an Array whose elements are provided
by elements. It performs the following steps when called:
The abstract operation LengthOfArrayLike takes argument obj (an Object) and returns either a
normal completion
containing a non-negative integer or a throw
completion. It returns the value of the "length" property of an
array-like object. It performs the following steps when called:
The abstract operation CreateListFromArrayLike takes argument obj (an ECMAScript language
value) and optional argument elementTypes (a List of names
of ECMAScript Language Types) and returns either a normal completion
containing a List of
ECMAScript language
values or a throw
completion. It is used to create a List value
whose elements are provided by the indexed properties of obj. elementTypes contains
the names of ECMAScript Language Types that are allowed for element values of the List that is
created. It performs the following steps when called:
1. If elementTypes is not present, set
elementTypes to « Undefined, Null, Boolean, String, Symbol, Number, BigInt, Object ».
The abstract operation OrdinaryHasInstance takes arguments C (an ECMAScript language
value) and O (an ECMAScript language
value) and returns either a normal
completion containing a Boolean or a throw
completion. It implements the default algorithm for determining if O inherits
from the instance object inheritance path provided by C. It performs the following steps when
called:
7.3.22 SpeciesConstructor ( O, defaultConstructor )
The abstract operation SpeciesConstructor takes arguments O (an Object) and
defaultConstructor (a constructor) and returns either a
normal completion
containing a constructor or a throw
completion. It is used to retrieve the constructor that should be used to
create new objects that are derived from O. defaultConstructor is the constructor to use if a constructor@@species property cannot be
found starting from O. It performs the following steps when called:
The target passed in here is always a newly created object which is not directly accessible in case of
an error being thrown.
7.3.26 PrivateElementFind ( O, P )
The abstract operation PrivateElementFind takes arguments O (an Object) and P (a
Private Name) and returns a
PrivateElement
or empty. It performs the following steps when called:
1. If O.[[PrivateElements]]
contains a PrivateElementpe such that pe.[[Key]] is P, then
3. If entry is not empty, throw a
TypeError exception.
4. Append PrivateElement
{ [[Key]]: P, [[Kind]]:
field, [[Value]]: value } to
O.[[PrivateElements]].
5. Return unused.
7.3.28 PrivateMethodOrAccessorAdd ( O, method )
The abstract operation PrivateMethodOrAccessorAdd takes arguments O (an Object) and
method (a PrivateElement)
and returns either a normal completion
containingunused or a throw
completion. It performs the following steps when called:
1. Assert: method.[[Kind]] is either method or
accessor.
7.3.33 InitializeInstanceElements ( O, constructor )
The abstract operation InitializeInstanceElements takes arguments O (an Object) and
constructor (an ECMAScript function object) and returns either
a normal completion
containingunused or a throw
completion. It performs the following steps when called:
1. Let methods be the value of constructor.[[PrivateMethods]].
The abstract operation IteratorComplete takes argument iterResult (an Object) and returns either
a normal completion
containing a Boolean or a throw
completion. It performs the following steps when called:
The abstract operation IteratorStep takes argument iteratorRecord (an Iterator Record) and returns
either a normal completion
containing either an Object or false, or a throw
completion. It requests the next value from iteratorRecord.[[Iterator]] by calling iteratorRecord.[[NextMethod]] and returns either false indicating that the
iterator has reached its end or the IteratorResult object if a next value is available. It performs the
following steps when called:
The abstract operation IteratorStepValue takes argument iteratorRecord (an Iterator Record) and returns
either a normal completion
containing either an ECMAScript language
value or done, or a throw
completion. It requests the next value from iteratorRecord.[[Iterator]] by calling iteratorRecord.[[NextMethod]] and returns either done indicating that the
iterator has reached its end or the value from the IteratorResult object if a next value is available. It
performs the following steps when called:
The abstract operation IteratorClose takes arguments iteratorRecord (an Iterator Record) and
completion (a Completion
Record) and returns a Completion
Record. It is used to notify an iterator that it should perform any actions it would
normally perform when it has reached its completed state. It performs the following steps when called:
The abstract operation AsyncIteratorClose takes arguments iteratorRecord (an Iterator Record) and
completion (a Completion
Record) and returns a Completion
Record. It is used to notify an async iterator that it should perform any actions it
would normally perform when it has reached its completed state. It performs the following steps when called:
7. If innerResult.[[Value]]is
not an Object, throw a
TypeError exception.
8. Return ? completion.
7.4.12 CreateIterResultObject ( value, done )
The abstract operation CreateIterResultObject takes arguments value (an ECMAScript language
value) and done (a Boolean) and returns an Object that conforms to the
IteratorResult interface. It creates an object that conforms to the IteratorResult interface.
It performs the following steps when called:
The abstract operation CreateListIteratorRecord takes argument list (a List of
ECMAScript language
values) and returns an Iterator Record. It creates an
Iterator (27.1.1.2) object record whose
next method returns the successive elements of list. It performs the following steps
when called:
1. Let closure be a new Abstract Closure with no
parameters that captures list and performs the following steps when called:
The definitions for this operation are distributed over the "ECMAScript Language" sections of this
specification. Each definition appears after the defining occurrence of the relevant productions.
"*default*" is used within this specification as a synthetic name for a module's
default export when it does not have another name. An entry in the module's [[Environment]] is created with that name and holds the corresponding value, and
resolving the export named "default" by calling ResolveExport (
exportName [ , resolveSet ] ) for the module will return a
ResolvedBinding Record
whose [[BindingName]] is "*default*", which will then
resolve in the module's [[Environment]] to the above-mentioned value. This is
done only for ease of specification, so that anonymous default exports can be resolved like any other
export. This "*default*" string is never accessible to ECMAScript code or to the
module linking algorithm.
It is defined piecewise over the following productions:
It is not necessary to treat export defaultAssignmentExpression as
a constant declaration because there is no syntax that permits assignment to the internal bound name
used to reference a module's default object.
8.2.4 Static Semantics: LexicallyDeclaredNames
The syntax-directed
operation LexicallyDeclaredNames takes no arguments and returns a List of
Strings. It is defined piecewise over the following productions:
The syntax-directed
operation LexicallyScopedDeclarations takes no arguments and returns a List of
Parse Nodes. It is defined
piecewise over the following productions:
The syntax-directed
operation VarDeclaredNames takes no arguments and returns a List of
Strings. It is defined piecewise over the following productions:
The syntax-directed
operation VarScopedDeclarations takes no arguments and returns a List of
Parse Nodes. It is defined
piecewise over the following productions:
The syntax-directed
operation TopLevelLexicallyDeclaredNames takes no arguments and returns a List of
Strings. It is defined piecewise over the following productions:
The syntax-directed
operation TopLevelLexicallyScopedDeclarations takes no arguments and returns a List of
Parse Nodes. It is defined
piecewise over the following productions:
The syntax-directed
operation TopLevelVarDeclaredNames takes no arguments and returns a List of
Strings. It is defined piecewise over the following productions:
The syntax-directed
operation TopLevelVarScopedDeclarations takes no arguments and returns a List of
Parse Nodes. It is defined
piecewise over the following productions:
The syntax-directed
operation ContainsDuplicateLabels takes argument labelSet (a List of
Strings) and returns a Boolean. It is defined piecewise over the following productions:
The syntax-directed
operation ContainsUndefinedBreakTarget takes argument labelSet (a List of
Strings) and returns a Boolean. It is defined piecewise over the following productions:
The syntax-directed
operation ContainsUndefinedContinueTarget takes arguments iterationSet (a
List of
Strings) and labelSet (a List of
Strings) and returns a Boolean. It is defined piecewise over the following productions:
The abstract operation IsAnonymousFunctionDefinition takes argument expr (an AssignmentExpressionParse Node, an InitializerParse Node, or an ExpressionParse Node) and returns a
Boolean. It determines if its argument is a function definition that does not bind a name. It performs the
following steps when called:
The syntax-directed
operation ComputedPropertyContains takes argument symbol (a grammar symbol)
and returns a Boolean. It is defined piecewise over the following productions:
undefined is passed for environment to indicate that a PutValue operation should be used
to assign the initialization value. This is the case for var statements and formal
parameter lists of some non-strict functions (See
10.2.11).
In those cases a lexical binding is hoisted and preinitialized prior to evaluation of its initializer.
It is defined piecewise over the following productions:
When undefined is passed for environment it indicates that a PutValue operation should be used
to assign the initialization value. This is the case for formal parameter lists of non-strict functions. In
that case the formal parameter bindings are preinitialized in order to deal with the possibility of
multiple parameters with the same name.
It is defined piecewise over the following productions:
The syntax-directed
operation AssignmentTargetType takes no arguments and returns
simple or invalid. It is defined piecewise over the following
productions:
Environment Record is a specification type used to
define the association of Identifiers to specific variables and
functions, based upon the lexical nesting structure of ECMAScript code. Usually an Environment Record is
associated with some specific syntactic structure of ECMAScript code such as a FunctionDeclaration, a BlockStatement, or
a Catch clause of a
TryStatement.
Each time such code is evaluated, a new Environment Record is created to record the identifier bindings that
are created by that code.
Every Environment Record has an [[OuterEnv]] field, which is either
null or a reference to an outer Environment Record. This is used to model the logical
nesting of Environment Record values. The outer reference of an (inner) Environment Record is a reference to
the Environment Record that logically surrounds the inner Environment Record. An outer Environment Record may,
of course, have its own outer Environment Record. An Environment Record may serve as the outer environment for
multiple inner Environment Records. For example, if a FunctionDeclaration contains
two nested FunctionDeclarations then the
Environment Records of each of the nested functions will have as their outer Environment Record the
Environment Record of the current evaluation of the surrounding function.
Environment Records are purely specification mechanisms and need not correspond to any specific artefact of
an ECMAScript implementation. It is impossible for an ECMAScript program to directly access or manipulate such
values.
A Function Environment
Record corresponds to the invocation of an ECMAScript function object, and
contains bindings for the top-level declarations within that function. It may establish a new
this binding. It also captures the state necessary to support super
method invocations.
An Object Environment
Record is used to define the effect of ECMAScript elements such as WithStatement
that
associate identifier bindings with the properties of some object.
A Global Environment
Record is used for Script global declarations. It
does not have an outer environment; its [[OuterEnv]] is
null. It may be prepopulated with identifier bindings and it includes an
associated global object whose
properties provide some of the global environment's identifier bindings. As ECMAScript code is
executed, additional properties may be added to the global
object and the initial properties may be modified.
The Environment Record abstract
class includes the abstract specification methods defined in Table
16. These abstract methods have distinct concrete algorithms for each of the concrete
subclasses.
Determine if an Environment Record
has a binding for the String value N. Return true if it does and
false if it does not.
CreateMutableBinding(N, D)
Create a new but uninitialized mutable binding in an Environment
Record. The String value N is the text of the bound name. If the
Boolean argument D is true the binding may be subsequently deleted.
CreateImmutableBinding(N, S)
Create a new but uninitialized immutable binding in an Environment
Record. The String value N is the text of the bound name. If
S is true then attempts to set it after it has been initialized will
always throw an exception, regardless of the strict mode setting of operations that reference that
binding.
InitializeBinding(N, V)
Set the value of an already existing but uninitialized binding in an Environment
Record. The String value N is the text of the bound name.
V is the value for the binding and is a value of any ECMAScript language
type.
SetMutableBinding(N, V, S)
Set the value of an already existing mutable binding in an Environment
Record. The String value N is the text of the bound name.
V is the value for the binding and may be a value of any ECMAScript language
type. Sis
a
Boolean flag. If S is true and the binding cannot
be set throw a TypeError exception.
GetBindingValue(N, S)
Returns the value of an already existing binding from an Environment
Record. The String value N is the text of the bound name.
S is used to identify references originating in strict mode
code or that otherwise require strict mode reference semantics. If S
is true and the binding does not exist throw a
ReferenceError exception. If the binding exists but is uninitialized a
ReferenceError is thrown, regardless of the value of S.
DeleteBinding(N)
Delete a binding from an Environment
Record. The String value N is the text of the bound name. If a
binding for N exists, remove the binding and return true. If the
binding exists but cannot be removed return false. If the binding does not
exist return true.
HasThisBinding()
Determine if an Environment Record
establishes a this binding. Return true if it does and
false if it does not.
HasSuperBinding()
Determine if an Environment Record
establishes a super method binding. Return true if it does and
false if it does not.
WithBaseObject()
If this Environment Record
is associated with a with statement, return the with object. Otherwise, return
undefined.
9.1.1.1 Declarative Environment Records
Each Declarative Environment Record
is associated with an ECMAScript program scope containing variable, constant, let, class, module, import,
and/or function declarations. A Declarative Environment Record binds the set of identifiers defined by the
declarations contained within its scope.
The behaviour of the concrete specification methods for Declarative Environment Records is defined by the
following algorithms.
9.1.1.1.1 HasBinding ( N )
The HasBinding concrete method of a Declarative
Environment RecordenvRec takes argument N (a String) and
returns a normal completion
containing a Boolean. It determines if the argument identifier is one of the
identifiers bound by the record. It performs the following steps when called:
1. If envRec has a binding for N, return
true.
2. Return false.
9.1.1.1.2 CreateMutableBinding ( N, D )
The CreateMutableBinding concrete method of a Declarative
Environment RecordenvRec takes arguments N (a String) and
D (a Boolean) and returns a normal
completion containingunused. It creates a new mutable binding
for the name N that is uninitialized. A binding must not already exist in this Environment Record for
N. If D is true, the new binding is marked as being subject to
deletion. It performs the following steps when called:
1. Assert: envRec does not
already have a binding for N.
2. Create a mutable binding in envRec for N
and record that it is uninitialized. If D is true, record that the
newly created binding may be deleted by a subsequent DeleteBinding call.
3. Return unused.
9.1.1.1.3 CreateImmutableBinding ( N, S )
The CreateImmutableBinding concrete method of a Declarative
Environment RecordenvRec takes arguments N (a String) and
S (a Boolean) and returns a normal
completion containingunused. It creates a new immutable
binding for the name N that is uninitialized. A binding must not already exist in this
Environment Record for
N. If S is true, the new binding is marked as a strict binding.
It performs the following steps when called:
1. Assert: envRec does not
already have a binding for N.
2. Create an immutable binding in envRec for
N and record that it is uninitialized. If S is true, record
that the newly created binding is a strict binding.
3. Return unused.
9.1.1.1.4 InitializeBinding ( N, V )
The InitializeBinding concrete method of a Declarative
Environment RecordenvRec takes arguments N (a String) and
V (an ECMAScript language
value) and returns a normal
completion containingunused. It is used to set the bound
value of the current binding of the identifier whose name is N to the value V. An
uninitialized binding for N must already exist. It performs the following steps when called:
1. Assert: envRec must have
an uninitialized binding for N.
2. Set the bound value for N in envRec to
V.
3. Record that the binding for
N in envRec has been initialized.
4. Return unused.
9.1.1.1.5 SetMutableBinding ( N, V, S )
The SetMutableBinding concrete method of a Declarative
Environment RecordenvRec takes arguments N (a String),
V (an ECMAScript language
value), and S (a Boolean) and returns either a normal completion
containingunused or a throw
completion. It attempts to change the bound value of the current binding of the
identifier whose name is N to the value V. A binding for N normally
already exists, but in rare cases it may not. If the binding is an immutable binding, a
TypeError is thrown if S is true. It performs the
following steps when called:
1. If
envRec does not have a binding for N, then
a. If S is true, throw a
ReferenceError exception.
b. Perform
! envRec.CreateMutableBinding(N, true).
c. Perform
! envRec.InitializeBinding(N, V).
d. Return unused.
2. If the binding for N in envRec is a
strict binding, set S to true.
3. If the binding for N in envRec has not
yet been initialized, then
a. Throw a ReferenceError exception.
4. Else if the binding for N in envRec is a
mutable binding, then
a. Change its bound value to V.
5. Else,
a. Assert: This is an attempt to
change the value of an immutable binding.
b. If S is true, throw a
TypeError exception.
6. Return unused.
Note
An example of ECMAScript code that results in a missing binding at step 1 is:
functionf() { eval("var x; x = (delete x, 0);"); }
9.1.1.1.6 GetBindingValue ( N, S )
The GetBindingValue concrete method of a Declarative
Environment RecordenvRec takes arguments N (a String) and
S (a Boolean) and returns either a normal
completion containing an ECMAScript
language value or a throw
completion. It returns the value of its bound identifier whose name is N.
If the binding exists but is uninitialized a ReferenceError is thrown, regardless of
the value of S. It performs the following steps when called:
2. If the binding for N in envRec is an
uninitialized binding, throw a ReferenceError exception.
3. Return the value currently bound to N in
envRec.
9.1.1.1.7 DeleteBinding ( N )
The DeleteBinding concrete method of a Declarative
Environment RecordenvRec takes argument N (a String) and
returns a normal completion
containing a Boolean. It can only delete bindings that have been explicitly
designated as being subject to deletion. It performs the following steps when called:
2. If the binding for N in envRec cannot be
deleted, return false.
3. Remove the binding for N from envRec.
4. Return true.
9.1.1.1.8 HasThisBinding ( )
The HasThisBinding concrete method of a Declarative
Environment RecordenvRec takes no arguments and returns
false. It performs the following steps when called:
The HasSuperBinding concrete method of a Declarative
Environment RecordenvRec takes no arguments and returns
false. It performs the following steps when called:
The WithBaseObject concrete method of a Declarative
Environment RecordenvRec takes no arguments and returns
undefined. It performs the following steps when called:
1. Return undefined.
9.1.1.2 Object Environment Records
Each Object Environment Record is
associated with an object called its binding object. An Object Environment Record binds the set
of string identifier names that directly correspond to the property names of its binding object. Property
keys that are not strings in the form of an IdentifierName are not included
in the set of bound identifiers. Both own and inherited properties are included in the set regardless of
the setting of their [[Enumerable]] attribute. Because properties can be
dynamically added and deleted from objects, the set of identifiers bound by an Object Environment Record
may potentially change as a side-effect of any operation that adds or deletes properties. Any bindings
that are created as a result of such a side-effect are considered to be a mutable binding even if the
Writable attribute of the corresponding property is false. Immutable bindings do not
exist for Object Environment Records.
Object Environment Records created for with statements (14.11) can
provide their binding object as an implicit this value for use in function calls. The
capability is controlled by a Boolean [[IsWithEnvironment]] field.
Object Environment Records have the additional state fields listed in Table
17.
Indicates whether this Environment
Record is created for a with statement.
The behaviour of the concrete specification methods for Object Environment Records is defined by the
following algorithms.
9.1.1.2.1 HasBinding ( N )
The HasBinding concrete method of an Object Environment
RecordenvRec takes argument N (a String) and returns either a
normal completion
containing a Boolean or a throw
completion. It determines if its associated binding object has a property whose name
is N. It performs the following steps when called:
1. Let bindingObject be envRec.[[BindingObject]].
2. Let foundBinding be ? HasProperty(bindingObject, N).
3. If foundBinding is false, return
false.
4. If envRec.[[IsWithEnvironment]] is false, return
true.
a. Let blocked be ToBoolean(?
Get(unscopables, N)).
b. If blocked is true, return
false.
7. Return true.
9.1.1.2.2 CreateMutableBinding ( N, D )
The CreateMutableBinding concrete method of an Object
Environment RecordenvRec takes arguments N (a String) and
D (a Boolean) and returns either a normal
completion containingunused or a throw
completion. It creates in an Environment
Record's associated binding object a property whose name is N and
initializes it to the value undefined. If D is true,
the new property's [[Configurable]] attribute is set to
true; otherwise it is set to false. It performs the following
steps when called:
1. Let bindingObject be envRec.[[BindingObject]].
2. Perform ? DefinePropertyOrThrow(bindingObject, N,
PropertyDescriptor { [[Value]]: undefined, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: D }).
3. Return unused.
Note
Normally envRec will not have a binding for N but if it does, the semantics
of DefinePropertyOrThrow
may result in an existing binding being replaced or shadowed or cause an abrupt
completion to be returned.
9.1.1.2.3 CreateImmutableBinding ( N, S )
The CreateImmutableBinding concrete method of an Object
Environment Record is never used within this specification.
In this specification, all uses of CreateMutableBinding for Object Environment
Records are immediately followed by a call to InitializeBinding for the same
name. Hence, this specification does not explicitly track the initialization state of bindings in
Object Environment
Records.
9.1.1.2.5 SetMutableBinding ( N, V, S )
The SetMutableBinding concrete method of an Object
Environment RecordenvRec takes arguments N (a String),
V (an ECMAScript language
value), and S (a Boolean) and returns either a normal completion
containingunused or a throw
completion. It attempts to set the value of the Environment Record's
associated binding object's property whose name is N to the value V. A property
named N normally already exists but if it does not or is not currently writable, error
handling is determined by S. It performs the following steps when called:
1. Let bindingObject be envRec.[[BindingObject]].
2. Let stillExists be ? HasProperty(bindingObject, N).
3. If stillExists is false and
S is true, throw a ReferenceError exception.
The GetBindingValue concrete method of an Object
Environment RecordenvRec takes arguments N (a String) and
S (a Boolean) and returns either a normal
completion containing an ECMAScript
language value or a throw
completion. It returns the value of its associated binding object's property whose
name is N. The property should already exist but if it does not the result depends upon
S. It performs the following steps when called:
The DeleteBinding concrete method of an Object
Environment RecordenvRec takes argument N (a String) and
returns either a normal completion
containing a Boolean or a throw
completion. It can only delete bindings that correspond to properties of the
environment object whose [[Configurable]] attribute have the value
true. It performs the following steps when called:
1. Let bindingObject be envRec.[[BindingObject]].
2. Return ? bindingObject.[[Delete]](N).
9.1.1.2.8 HasThisBinding ( )
The HasThisBinding concrete method of an Object
Environment RecordenvRec takes no arguments and returns
false. It performs the following steps when called:
The HasSuperBinding concrete method of an Object
Environment RecordenvRec takes no arguments and returns
false. It performs the following steps when called:
The WithBaseObject concrete method of an Object
Environment RecordenvRec takes no arguments and returns an Object or
undefined. It performs the following steps when called:
1. If envRec.[[IsWithEnvironment]] is true, return
envRec.[[BindingObject]].
2. Otherwise, return undefined.
9.1.1.3 Function Environment Records
A Function Environment Record is a
Declarative Environment
Record that is used to represent the top-level scope of a function and, if the function
is not an ArrowFunction, provides a
this binding. If a function is not an ArrowFunction function and
references super, its Function Environment Record also contains the state that is used to
perform super method invocations from within the function.
Function Environment Records have the additional state fields listed in Table
18.
If this Environment
Record was created by the [[Construct]] internal
method, [[NewTarget]] is the value of the [[Construct]]newTarget parameter. Otherwise, its value is
undefined.
Function Environment Records support all of the Declarative
Environment Record methods listed in Table
16 and share the same specifications for all of those methods except for HasThisBinding
and HasSuperBinding. In addition, Function Environment Records support the methods listed in Table
19:
Set the [[ThisValue]] and record that it has been initialized.
GetThisBinding()
Return the value of this Environment
Record's this binding. Throws a
ReferenceError if the this binding has not been initialized.
GetSuperBase()
Return the object that is the base for super property accesses bound in this
Environment
Record. The value undefined indicates that such accesses
will produce runtime errors.
The behaviour of the additional concrete specification methods for Function Environment Records is
defined by the following algorithms:
1. Assert: envRec.[[ThisBindingStatus]] is not lexical.
2. If envRec.[[ThisBindingStatus]] is initialized, throw a
ReferenceError exception.
3. Set envRec.[[ThisValue]] to
V.
4. Set envRec.[[ThisBindingStatus]] to initialized.
5. Return V.
9.1.1.3.2 HasThisBinding ( )
The HasThisBinding concrete method of a Function
Environment RecordenvRec takes no arguments and returns a Boolean. It
performs the following steps when called:
1. If envRec.[[ThisBindingStatus]] is lexical, return
false; otherwise, return true.
9.1.1.3.3 HasSuperBinding ( )
The HasSuperBinding concrete method of a Function
Environment RecordenvRec takes no arguments and returns a Boolean. It
performs the following steps when called:
1. If envRec.[[ThisBindingStatus]] is lexical, return
false.
2. If envRec.[[FunctionObject]].[[HomeObject]] is
undefined, return false; otherwise, return
true.
A Global Environment Record is used to
represent the outer most scope that is shared by all of the ECMAScript Script elements that are processed in a
common realm. A Global Environment Record provides
the bindings for built-in globals (clause 19), properties of the global
object, and for all
top-level declarations (8.2.9,
8.2.11)
that occur within a Script.
Determines if the argument is the name of a global
object property that may not be shadowed by a global lexical binding.
CanDeclareGlobalVar (N)
Determines if a corresponding CreateGlobalVarBinding call would succeed if called for the same
argument N.
CanDeclareGlobalFunction (N)
Determines if a corresponding CreateGlobalFunctionBinding call would succeed if called for the
same argument N.
CreateGlobalVarBinding(N, D)
Used to create and initialize to undefined a global var binding
in the [[ObjectRecord]] component of a Global Environment
Record. The binding will be a mutable binding. The corresponding global object property
will have attribute values appropriate for a var. The String value N is
the bound name. If D is true, the binding may be deleted.
Logically equivalent to CreateMutableBinding followed by a SetMutableBinding but it allows var
declarations to receive special treatment.
CreateGlobalFunctionBinding(N, V, D)
Create and initialize a global function binding in the [[ObjectRecord]] component of a Global Environment
Record. The binding will be a mutable binding. The corresponding global object property
will have attribute values appropriate for a function. The String value
N is the bound name. V is the initialization value. If the Boolean
argument D is true, the binding may be deleted. Logically
equivalent to CreateMutableBinding followed by a SetMutableBinding but it allows function
declarations to receive special treatment.
The behaviour of the concrete specification methods for Global Environment Records is defined by the
following algorithms.
9.1.1.4.1 HasBinding ( N )
The HasBinding concrete method of a Global Environment
RecordenvRec takes argument N (a String) and returns either a
normal completion
containing a Boolean or a throw
completion. It determines if the argument identifier is one of the identifiers bound
by the record. It performs the following steps when called:
1. Let DclRec be envRec.[[DeclarativeRecord]].
2. If ! DclRec.HasBinding(N) is
true, return true.
3. Let ObjRec be envRec.[[ObjectRecord]].
4. Return ? ObjRec.HasBinding(N).
9.1.1.4.2 CreateMutableBinding ( N, D )
The CreateMutableBinding concrete method of a Global
Environment RecordenvRec takes arguments N (a String) and
D (a Boolean) and returns either a normal
completion containingunused or a throw
completion. It creates a new mutable binding for the name N that is
uninitialized. The binding is created in the associated DeclarativeRecord. A binding for N
must not already exist in the DeclarativeRecord. If D is true, the new
binding is marked as being subject to deletion. It performs the following steps when called:
1. Let DclRec be envRec.[[DeclarativeRecord]].
2. If ! DclRec.HasBinding(N) is
true, throw a TypeError exception.
3. Return
! DclRec.CreateMutableBinding(N, D).
9.1.1.4.3 CreateImmutableBinding ( N, S )
The CreateImmutableBinding concrete method of a Global
Environment RecordenvRec takes arguments N (a String) and
S (a Boolean) and returns either a normal
completion containingunused or a throw
completion. It creates a new immutable binding for the name N that is
uninitialized. A binding must not already exist in this Environment
Record for N. If S is true, the new binding
is marked as a strict binding. It performs the following steps when called:
1. Let DclRec be envRec.[[DeclarativeRecord]].
2. If ! DclRec.HasBinding(N) is
true, throw a TypeError exception.
3. Return
! DclRec.CreateImmutableBinding(N, S).
9.1.1.4.4 InitializeBinding ( N, V )
The InitializeBinding concrete method of a Global
Environment RecordenvRec takes arguments N (a String) and
V (an ECMAScript language
value) and returns either a normal completion
containingunused or a throw
completion. It is used to set the bound value of the current binding of the
identifier whose name is N to the value V. An uninitialized binding for
N must already exist. It performs the following steps when called:
The SetMutableBinding concrete method of a Global
Environment RecordenvRec takes arguments N (a String),
V (an ECMAScript language
value), and S (a Boolean) and returns either a normal completion
containingunused or a throw
completion. It attempts to change the bound value of the current binding of the
identifier whose name is N to the value V. If the binding is an immutable binding
and S is true, a TypeError is thrown. A property named
N normally already exists but if it does not or is not currently writable, error handling is
determined by S. It performs the following steps when called:
1. Let DclRec be envRec.[[DeclarativeRecord]].
2. If ! DclRec.HasBinding(N) is
true, then
a. Return
? DclRec.SetMutableBinding(N, V, S).
3. Let ObjRec be envRec.[[ObjectRecord]].
4. Return ? ObjRec.SetMutableBinding(N, V,
S).
9.1.1.4.6 GetBindingValue ( N, S )
The GetBindingValue concrete method of a Global
Environment RecordenvRec takes arguments N (a String) and
S (a Boolean) and returns either a normal
completion containing an ECMAScript
language value or a throw
completion. It returns the value of its bound identifier whose name is N.
If the binding is an uninitialized binding throw a ReferenceError exception. A
property named N normally already exists but if it does not or is not currently writable,
error handling is determined by S. It performs the following steps when called:
1. Let DclRec be envRec.[[DeclarativeRecord]].
2. If ! DclRec.HasBinding(N) is
true, then
a. Return
? DclRec.GetBindingValue(N, S).
3. Let ObjRec be envRec.[[ObjectRecord]].
4. Return ? ObjRec.GetBindingValue(N, S).
9.1.1.4.7 DeleteBinding ( N )
The DeleteBinding concrete method of a Global
Environment RecordenvRec takes argument N (a String) and
returns either a normal completion
containing a Boolean or a throw
completion. It can only delete bindings that have been explicitly designated as being
subject to deletion. It performs the following steps when called:
1. Let DclRec be envRec.[[DeclarativeRecord]].
2. If ! DclRec.HasBinding(N) is
true, then
a. Return
! DclRec.DeleteBinding(N).
3. Let ObjRec be envRec.[[ObjectRecord]].
4. Let globalObject be ObjRec.[[BindingObject]].
5. Let existingProp be ? HasOwnProperty(globalObject, N).
6. If existingProp is true, then
a. Let status be ? ObjRec.DeleteBinding(N).
b. If status is true and
envRec.[[VarNames]] contains N, then
i. Remove N from envRec.[[VarNames]].
c. Return status.
7. Return true.
9.1.1.4.8 HasThisBinding ( )
The HasThisBinding concrete method of a Global
Environment RecordenvRec takes no arguments and returns
true. It performs the following steps when called:
The HasSuperBinding concrete method of a Global
Environment RecordenvRec takes no arguments and returns
false. It performs the following steps when called:
The WithBaseObject concrete method of a Global
Environment RecordenvRec takes no arguments and returns
undefined. It performs the following steps when called:
The HasVarDeclaration concrete method of a Global
Environment RecordenvRec takes argument N (a String) and
returns a Boolean. It determines if the argument identifier has a binding in this record that was
created using a VariableStatement or a
FunctionDeclaration. It
performs the following steps when called:
1. Let varDeclaredNames be envRec.[[VarNames]].
2. If varDeclaredNames contains N, return
true.
3. Return false.
9.1.1.4.13 HasLexicalDeclaration ( N )
The HasLexicalDeclaration concrete method of a Global
Environment RecordenvRec takes argument N (a String) and
returns a Boolean. It determines if the argument identifier has a binding in this record that was
created using a lexical declaration such as a LexicalDeclaration or a
ClassDeclaration. It
performs the following steps when called:
1. Let DclRec be envRec.[[DeclarativeRecord]].
2. Return ! DclRec.HasBinding(N).
9.1.1.4.14 HasRestrictedGlobalProperty ( N )
The HasRestrictedGlobalProperty concrete method of a Global
Environment RecordenvRec takes argument N (a String) and
returns either a normal completion
containing a Boolean or a throw
completion. It determines if the argument identifier is the name of a property of the
global object that must not be
shadowed by a global lexical binding. It performs the following steps when called:
1. Let ObjRec be envRec.[[ObjectRecord]].
2. Let globalObject be ObjRec.[[BindingObject]].
3. Let existingProp be ? globalObject.[[GetOwnProperty]](N).
4. If existingProp is undefined,
return false.
5. If existingProp.[[Configurable]] is true, return false.
6. Return true.
Note
Properties may exist upon a global object that were
directly created rather than being declared using a var or function declaration. A global lexical
binding may not be created that has the same name as a non-configurable property of the global object. The global
property "undefined" is an example of such a property.
9.1.1.4.15 CanDeclareGlobalVar ( N )
The CanDeclareGlobalVar concrete method of a Global
Environment RecordenvRec takes argument N (a String) and
returns either a normal completion
containing a Boolean or a throw
completion. It determines if a corresponding CreateGlobalVarBinding call would
succeed if called for the same argument N. Redundant var declarations and var declarations
for pre-existing global object properties are
allowed. It performs the following steps when called:
1. Let ObjRec be envRec.[[ObjectRecord]].
2. Let globalObject be ObjRec.[[BindingObject]].
3. Let hasProperty be ? HasOwnProperty(globalObject, N).
The CanDeclareGlobalFunction concrete method of a Global
Environment RecordenvRec takes argument N (a String) and
returns either a normal completion
containing a Boolean or a throw
completion. It determines if a corresponding CreateGlobalFunctionBinding call would
succeed if called for the same argument N. It performs the following steps when called:
1. Let ObjRec be envRec.[[ObjectRecord]].
2. Let globalObject be ObjRec.[[BindingObject]].
3. Let existingProp be ? globalObject.[[GetOwnProperty]](N).
4. If existingProp is undefined,
return ? IsExtensible(globalObject).
5. If existingProp.[[Configurable]] is true, return true.
6. If IsDataDescriptor(existingProp)
is true and existingProp has attribute values { [[Writable]]: true, [[Enumerable]]: true }, return
true.
7. Return false.
9.1.1.4.17 CreateGlobalVarBinding ( N, D )
The CreateGlobalVarBinding concrete method of a Global
Environment RecordenvRec takes arguments N (a String) and
D (a Boolean) and returns either a normal
completion containingunused or a throw
completion. It creates and initializes a mutable binding in the associated Object Environment
Record and records the bound name in the associated [[VarNames]]List. If
a binding already exists, it is reused and assumed to be initialized. It performs the following steps
when called:
1. Let ObjRec be envRec.[[ObjectRecord]].
2. Let globalObject be ObjRec.[[BindingObject]].
3. Let hasProperty be ? HasOwnProperty(globalObject, N).
4. Let extensible be ? IsExtensible(globalObject).
5. If hasProperty is false and
extensible is true, then
a. Perform ? ObjRec.CreateMutableBinding(N, D).
b. Perform ? ObjRec.InitializeBinding(N,
undefined).
6. If envRec.[[VarNames]] does
not contain N, then
a. Append N to envRec.[[VarNames]].
7. Return unused.
9.1.1.4.18 CreateGlobalFunctionBinding ( N, V,
D )
8. If envRec.[[VarNames]] does
not contain N, then
a. Append N to envRec.[[VarNames]].
9. Return unused.
Note
Global function declarations are always represented as own properties of the global object. If
possible, an existing own property is reconfigured to have a standard set of attribute values. Step
7 is
equivalent to what calling the InitializeBinding concrete method would do and if
globalObject is a Proxy will produce the same sequence of Proxy trap calls.
9.1.1.5 Module Environment Records
A Module Environment Record is a Declarative Environment
Record that is used to represent the outer scope of an ECMAScript Module. In additional
to normal mutable and immutable bindings, Module Environment Records also provide immutable import
bindings which are bindings that provide indirect access to a target binding that exists in another
Environment Record.
Module Environment Records support all of the Declarative
Environment Record methods listed in Table
16 and share the same specifications for all of those methods except for
GetBindingValue, DeleteBinding, HasThisBinding and GetThisBinding. In addition, Module Environment Records
support the methods listed in Table
22:
The behaviour of the additional concrete specification methods for Module Environment Records are defined
by the following algorithms:
9.1.1.5.1 GetBindingValue ( N, S )
The GetBindingValue concrete method of a Module
Environment RecordenvRec takes arguments N (a String) and
S (a Boolean) and returns either a normal
completion containing an ECMAScript
language value or a throw
completion. It returns the value of its bound identifier whose name is N.
However, if the binding is an indirect binding the value of the target binding is returned. If the
binding exists but is uninitialized a ReferenceError is thrown. It performs the
following steps when called:
The HasThisBinding concrete method of a Module
Environment RecordenvRec takes no arguments and returns
true. It performs the following steps when called:
The CreateImportBinding concrete method of a Module
Environment RecordenvRec takes arguments N (a String),
M (a Module Record), and
N2 (a String) and returns unused. It creates a new initialized
immutable indirect binding for the name N. A binding must not already exist in this Environment Record for
N. N2 is the name of a binding that exists in M's Module Environment
Record. Accesses to the value of the new binding will indirectly access the bound
value of the target binding. It performs the following steps when called:
1. Assert: envRec does not
already have a binding for N.
2. Assert: When M.[[Environment]] is instantiated, it will have a direct binding for
N2.
3. Create an immutable indirect binding in envRec for
N that references M and N2 as its target binding and record that
the binding is initialized.
The abstract operation NewObjectEnvironment takes arguments O (an Object), W (a
Boolean), and E (an Environment Record or
null) and returns an Object Environment
Record. It performs the following steps when called:
The abstract operation NewFunctionEnvironment takes arguments F (an ECMAScript function
object) and newTarget (an Object or undefined) and
returns a Function Environment
Record. It performs the following steps when called:
3. If F.[[ThisMode]] is
lexical, set env.[[ThisBindingStatus]] to
lexical.
4. Else, set env.[[ThisBindingStatus]] to uninitialized.
5. Set env.[[NewTarget]] to
newTarget.
6. Set env.[[OuterEnv]] to
F.[[Environment]].
7. Return env.
9.1.2.5 NewGlobalEnvironment ( G, thisValue )
The abstract operation NewGlobalEnvironment takes arguments G (an Object) and
thisValue (an Object) and returns a Global Environment
Record. It performs the following steps when called:
The abstract operation ResolvePrivateIdentifier takes arguments privEnv (a PrivateEnvironment
Record) and identifier (a String) and returns a Private
Name. It performs the
following steps when called:
Before it is evaluated, all ECMAScript code must be associated with a realm. Conceptually, a realm consists of a set of intrinsic objects, an
ECMAScript global environment, all of the ECMAScript code that is loaded within the scope of that global
environment, and other associated state and resources.
A realm
is represented in this specification as a Realm
Record with the fields specified in Table 24:
Template objects are canonicalized separately for each realm using its Realm
Record's [[TemplateMap]]. Each [[Site]] value is a Parse
Node that is a TemplateLiteral. The
associated [[Array]] value is the corresponding template object that is
passed to a tag function.
Note 1
Once a Parse Node becomes
unreachable, the corresponding [[Array]] is also unreachable, and it
would be unobservable if an implementation removed the pair from the [[TemplateMap]] list.
A map from the specifier strings imported by this realm to the resolved Module Record.
The list does not contain two different Records
with the same [[Specifier]].
2. Set fields of
realmRec.[[Intrinsics]] with the values listed in Table 6. The
field names are the names listed in column one of the table. The value of each field is a new object
value fully and recursively populated with property values as defined by the specification of each
object in clauses 19 through 28. All object property values
are newly created object values. All values that are built-in function objects
are created by performing CreateBuiltinFunction(steps,
length, name, slots, realmRec, prototype) where
steps is the definition of that function provided by this specification, name is
the initial value of the function's "name" property, length is the initial
value of the function's "length" property, slots is a list of the names,
if any, of the function's specified internal slots, and prototype is the specified value of
the function's [[Prototype]] internal slot. The creation of the intrinsics and
their properties must be ordered to avoid any dependencies upon objects that have not yet been created.
The abstract operation SetRealmGlobalObject takes arguments realmRec (a Realm
Record), globalObj (an Object or undefined), and
thisValue (an Object or undefined) and returns unused.
It performs the following steps when called:
2. For each property of the Global Object specified in clause
19, do
a. Let name be the String value of the property
name.
b. Let desc be the fully populated data Property
Descriptor for the property, containing the specified attributes for the
property. For properties listed in 19.2,
19.3,
or 19.4
the value of the [[Value]] attribute is the corresponding intrinsic object
from realmRec.
An execution context is a specification device that is
used to track the runtime evaluation of code by an ECMAScript implementation. At any point in time, there is
at most one execution context per agent that is actually executing code. This is
known as the agent's running execution context. All references to the
running execution context in
this specification denote the running execution context of
the surrounding agent.
The execution context
stack is used to track execution contexts. The running execution
context is always the top element of this stack. A new execution context is created
whenever control is transferred from the executable code associated with the currently running execution context to
executable code that is not associated with that execution context. The newly created execution context is
pushed onto the stack and becomes the running execution context.
An execution context contains whatever implementation specific state is necessary to track the execution
progress of its associated code. Each execution context has at least the state components listed in Table
25.
Table 25: State Components for All Execution Contexts
Component
Purpose
code evaluation state
Any state needed to perform, suspend, and resume evaluation of the code associated with this
execution context.
Evaluation of code by the running execution context
may be suspended at various points defined within this specification. Once the running execution context
has been suspended a different execution context may become the running execution
context and commence evaluating its code. At some later time a suspended execution context
may again become the running execution context
and continue evaluating its code at the point where it had previously been suspended. Transition of the
running execution context
status among execution contexts usually occurs in stack-like last-in/first-out manner. However, some
ECMAScript features require non-LIFO transitions of the running execution
context.
In most situations only the running execution context
(the top of the execution context stack) is
directly manipulated by algorithms within this specification. Hence when the terms “LexicalEnvironment”, and
“VariableEnvironment” are used without qualification they are in reference to those components of the
running execution context.
An execution context is purely a specification mechanism and need not correspond to any particular artefact
of an ECMAScript implementation. It is impossible for ECMAScript code to directly access or observe an
execution context.
9.4.1 GetActiveScriptOrModule ( )
The abstract operation GetActiveScriptOrModule takes no arguments and returns a Script
Record, a Module Record, or
null. It is used to determine the running script or module, based on the running execution context.
It performs the following steps when called:
3. If no such execution
context exists, return null. Otherwise, return ec's
ScriptOrModule.
9.4.2 ResolveBinding ( name [ , env ] )
The abstract operation ResolveBinding takes argument name (a String) and optional argument
env (an Environment Record or
undefined) and returns either a normal completion
containing a Reference
Record or a throw
completion. It is used to determine the binding of name. env can be
used to explicitly provide the Environment Record that is
to be searched for the binding. It performs the following steps when called:
1. If env is not present or env is
undefined, then
3. If the source
text matched by the syntactic production that is being evaluated is contained in
strict mode code, let
strict be true; else let strict be false.
The result of ResolveBinding is always a Reference
Record whose [[ReferencedName]] field is name.
9.4.3 GetThisEnvironment ( )
The abstract operation GetThisEnvironment takes no arguments and returns an Environment Record. It finds
the Environment Record that
currently supplies the binding of the keywordthis. It performs the following steps when called:
The abstract operation GetNewTarget takes no arguments and returns an Object or
undefined. It determines the NewTarget value using the LexicalEnvironment of the
running execution context.
It performs the following steps when called:
The abstract operation GetGlobalObject takes no arguments and returns an Object. It returns the global
object used by the currently running execution
context. It performs the following steps when called:
A Job is an Abstract Closure
with no parameters that initiates an ECMAScript computation when no other ECMAScript computation is currently
in progress.
At some future point in time, when there is no running context in the agent for which the job is
scheduled and that agent's execution context
stack is
empty, the implementation must:
Host
environments are not required
to treat Jobs
uniformly with respect to scheduling. For
example, web browsers and Node.js treat Promise-handling Jobs as a higher priority than other work;
future features may add Jobs that are not treated at such a high
priority.
At any particular time, scriptOrModule (a Script Record, a Module Record, or
null) is the active script or module
if all of the following conditions are true:
The specific choice of Realm is up to the host
environment. This initial execution context and
Realm
is only in use before any callback function is invoked. When a callback function related to a Job, like a
Promise handler, is invoked, the invocation pushes its own execution
context and Realm.
Particular kinds of Jobs have additional conformance requirements.
The WHATWG HTML specification (https://html.spec.whatwg.org/), for example, uses the
host-defined value to propagate the
incumbent settings object for Promise callbacks.
JobCallback Records have the fields listed in Table 28.
An implementation of HostMakeJobCallback must conform to the following requirements:
It must return a JobCallback Record whose
[[Callback]] field is callback.
The default implementation of HostMakeJobCallback performs the following steps when called:
1. Return the JobCallback
Record { [[Callback]]: callback, [[HostDefined]]: empty }.
ECMAScript hosts that are not web browsers must use the
default implementation of HostMakeJobCallback.
Note
This is called at the time that the callback is passed to the function that is responsible for its
being eventually scheduled and run. For example, promise.then(thenAction) calls
MakeJobCallback on thenAction at the time of invoking Promise.prototype.then,
not at the time of scheduling the reaction Job.
ECMAScript hosts that are not web browsers must use the
default implementation of HostCallJobCallback.
9.5.4 HostEnqueueGenericJob ( job, realm )
The host-defined abstract operation
HostEnqueueGenericJob takes arguments job (a JobAbstract
Closure) and realm (a Realm Record) and returns
unused. It schedules job in the realmrealm in the agent signified
by realm.[[AgentSignifier]] to be performed at some future time. The
Abstract Closures used with
this algorithm are intended to be scheduled without additional constraints, such as priority and ordering.
An implementation of HostEnqueueGenericJob must conform to the requirements in 9.5.
9.5.5 HostEnqueuePromiseJob ( job, realm )
The host-defined abstract operation
HostEnqueuePromiseJob takes arguments job (a JobAbstract
Closure) and realm (a Realm Record or
null) and returns unused. It schedules job to be
performed at some future time. The Abstract Closures used with
this algorithm are intended to be related to the handling of Promises, or otherwise, to be scheduled with
equal priority to Promise handling operations.
An implementation of HostEnqueuePromiseJob must conform to the requirements in 9.5 as well as the
following:
Let scriptOrModule be GetActiveScriptOrModule()
at the time HostEnqueuePromiseJob is invoked. If realm is not null, each
time job is invoked the implementation must perform implementation-defined
steps such that scriptOrModule is the active script or
module at the time of job's invocation.
Jobs
must run in the same order as the HostEnqueuePromiseJob invocations that scheduled them.
Note
The realm for Jobs returned by NewPromiseResolveThenableJob
is usually the result of calling GetFunctionRealm on the
thenfunction object. The
realm for Jobs returned by NewPromiseReactionJob
is usually the result of calling GetFunctionRealm on the
handler if the handler is not undefined. If the handler is
undefined, realm is null. For both kinds of Jobs, when
GetFunctionRealm completes
abnormally (i.e. called on a revoked Proxy), realm is the current Realm
Record at the time of the GetFunctionRealm call. When
the realm is null, no user ECMAScript code will be evaluated and no new
ECMAScript objects (e.g. Error objects) will be created. The WHATWG HTML specification (https://html.spec.whatwg.org/), for example, uses
realm to check for the ability to run script and for the entry concept.
The host-defined abstract operation
HostEnqueueTimeoutJob takes arguments timeoutJob (a JobAbstract
Closure), realm (a Realm Record), and
milliseconds (a non-negative finite Number) and returns
unused. It schedules timeoutJob in the realmrealm in the
agent
signified by realm.[[AgentSignifier]] to be performed after at least
milliseconds milliseconds.
An implementation of HostEnqueueTimeoutJob must conform to the requirements in 9.5.
9.6 InitializeHostDefinedRealm ( )
The abstract operation InitializeHostDefinedRealm takes no arguments and returns either a normal completion
containingunused or a throw
completion. It performs the following steps when called:
8. If the host requires that the this
binding in realm's global scope return an object other than the global
object, let
thisValue be such an object created in a host-defined manner. Otherwise, let
thisValue be undefined, indicating that realm's global
this binding should be the global object.
The default value computed for the isLittleEndian parameter when it is needed by the
algorithms GetValueFromBuffer
and SetValueInBuffer. The
choice is implementation-defined
and should be the alternative that is most efficient for the implementation. Once the value has been
observed it cannot change.
Initially a new empty List,
representing the list of objects and/or symbols to be kept alive until the end of the current
Job
Once the values of [[Signifier]], [[IsLockFree1]], and [[IsLockFree2]] have been observed by any agent in the agent
cluster they cannot change.
Note 2
The values of [[IsLockFree1]] and [[IsLockFree2]] are
not necessarily determined by the hardware, but may also reflect implementation choices that can vary over
time and between ECMAScript implementations.
There is no [[IsLockFree4]] field: 4-byte atomic operations are always
lock-free.
In practice, if an atomic operation is implemented with any type of lock the operation is not lock-free.
Lock-free does not imply wait-free: there is no upper bound on how many machine steps may be required to
complete a lock-free atomic operation.
That an atomic access of size n is lock-free does not imply anything about the (perceived)
atomicity of non-atomic accesses of size n, specifically, non-atomic accesses may still be
performed as a sequence of several separate memory accesses. See ReadSharedMemory and
WriteSharedMemory
for details.
Note 3
An agent
is a specification mechanism and need
not correspond to any particular artefact of an ECMAScript implementation.
9.7.1 AgentSignifier ( )
The abstract operation AgentSignifier takes no arguments and returns an agent signifier. It
performs the following steps when called:
In some environments it may not be reasonable for a given agent to suspend. For example, in a web
browser environment, it may be reasonable to disallow suspending a document's main event handling
thread, while still allowing workers' event handling threads to suspend.
9.8 Agent Clusters
An agent cluster is a maximal set of agents that can
communicate by operating on shared memory.
Note 1
Programs within different agents may share memory by unspecified
means. At a minimum, the backing memory for SharedArrayBuffers can be shared among the agents in the
cluster.
There may be agents that can communicate by message
passing that cannot share memory; they are never in the same agent cluster.
The agents in a cluster need not all be alive at
some particular point in time. If agentA creates another agentB, after which A terminates and B creates agentC, the three agents are in
the same cluster if A could share some memory with B and B could share some memory
with C.
All agents
within a cluster must have the same value
for the [[LittleEndian]] field in their respective Agent Records.
Note 3
If different agents within an agent cluster have
different values of [[LittleEndian]] it becomes hard to use shared memory for
multi-byte data.
All agents
within a cluster must have the same
values for the [[IsLockFree1]] field in their respective Agent
Records; similarly for the [[IsLockFree2]] field.
All agents
within a cluster must have different
values for the [[Signifier]] field in their respective Agent Records.
An embedding may deactivate (stop forward progress) or activate (resume forward progress) an agent without the
agent's
knowledge or cooperation. If the embedding does so, it must not leave some agents in the cluster active
while other agents in the cluster are deactivated
indefinitely.
Note 4
The purpose of the preceding restriction is to avoid a situation where an agent deadlocks or starves
because another agent has been deactivated. For example, if
an HTML shared worker that has a lifetime independent of documents in any windows were allowed to share
memory with the dedicated worker of such an independent document, and the document and its dedicated
worker were to be deactivated while the dedicated worker holds a lock (say, the document is pushed into
its window's history), and the shared worker then tries to acquire the lock, then the shared worker will
be blocked until the dedicated worker is activated again, if ever. Meanwhile other workers trying to
access the shared worker from other windows will starve.
The implication of the restriction is that it will not be possible to share memory between agents that
don't belong to the same suspend/wake collective within the embedding.
An embedding may terminate an agent without any of the agent's cluster's other
agents'
prior knowledge or cooperation. If an agent is terminated not by programmatic action
of its own or of another agent in the cluster but by forces external to
the cluster, then the embedding must choose one of two strategies: Either terminate all the agents in the
cluster, or provide reliable APIs that allow the agents in the cluster to coordinate so that at
least one remaining member of the cluster will be able to detect the termination, with the termination data
containing enough information to identify the agent that was terminated.
Note 5
Examples of that type of termination are: operating systems or users terminating agents that are running in
separate processes; the embedding itself terminating an agent that is running in-process with the
other agents when per-agent resource accounting
indicates that the agent is runaway.
Each of the following specification values, and values transitively reachable from them, belong to exactly
one agent cluster.
An agent cluster is a specification mechanism and need not correspond to any particular artefact of an
ECMAScript implementation.
9.9 Forward Progress
For an agent
to make forward progress is for
it to perform an evaluation step according to this specification.
An agent
becomes blocked when its running execution context
waits synchronously and indefinitely for an external event. Only agents whose Agent Record's [[CanBlock]] field is true can become blocked in this sense. An
unblockedagent is one that is not blocked.
Implementations must ensure that:
every unblocked agent with a dedicated executing
thread eventually makes forward progress
an agent
does not cause another agent to become
blocked except via explicit APIs that provide blocking.
Note
This, along with the liveness guarantee in the memory model, ensures that all
seq-cst writes eventually become observable to all agents.
9.10 Processing Model of WeakRef and FinalizationRegistry Targets
9.10.1 Objectives
This specification does not make any guarantees that any object or symbol will be garbage collected.
Objects or symbols which are not live may be released after long periods
of time, or never at all. For this reason, this specification uses the term "may" when describing behaviour
triggered by garbage collection.
The semantics of WeakRefs and FinalizationRegistrys
is based on two operations which happen at particular points in time:
When WeakRef.prototype.deref is called, the referent (if undefined is not
returned) is kept alive so that subsequent, synchronous accesses also return the same value. This list is
reset when synchronous work is done using the ClearKeptObjects abstract
operation.
Some ECMAScript implementations include garbage collector implementations which run in the background,
including when ECMAScript is idle. Letting the host environment schedule CleanupFinalizationRegistry
allows it to resume ECMAScript execution in order to run finalizer work, which may free up held values,
reducing overall memory usage.
9.10.2 Liveness
For some set of objects and/or symbols S a hypothetical
WeakRef-oblivious execution with respect to S is an execution whereby the abstract
operation WeakRefDeref of a WeakRef whose referent is
an element of S always returns undefined.
Note 1
WeakRef-obliviousness,
together with liveness, capture two notions. One, that a WeakRef
itself does not keep its referent alive. Two, that cycles in liveness does not imply that a value is live.
To be concrete, if determining v's liveness depends on determining the liveness of a WeakRef referent,
r, r's liveness cannot assume v's liveness, which would be circular
reasoning.
Note 2
WeakRef-obliviousness is
defined on sets of objects or symbols instead of individual values to account for cycles. If it were
defined on individual values, then a WeakRef referent in a
cycle will be considered live even though its identity is only observed via other WeakRef referents in the
cycle.
Note 3
Colloquially, we say that an individual object or symbol is live if every set containing it is live.
At any point during evaluation, a set of objects and/or symbols S is considered live if either of the following conditions is met:
Any element in S is included in any agent's [[KeptAlive]]List.
There exists a valid future hypothetical WeakRef-oblivious execution with respect to S that
observes the identity of any value in S.
Note 4
The second condition above intends to capture the intuition that a value is live if its identity is
observable via non-WeakRef means. A value's
identity may be observed by observing a strict equality comparison or observing the value being used as
key in a Map.
Note 5
Presence of an object or a symbol in a field, an internal slot, or a property does not imply that the
value is live. For example if the value in question is never passed back to the program, then it cannot
be observed.
This is the case for keys in a WeakMap, members of a WeakSet, as well as the [[WeakRefTarget]] and [[UnregisterToken]] fields of a
FinalizationRegistry
Cell record.
The above definition implies that, if a key in a WeakMap is not live, then its corresponding value is
not necessarily live either.
Note 6
Liveness is the lower bound for guaranteeing which WeakRefs
engines must not empty. Liveness as defined here is undecidable. In practice, engines use conservative
approximations such as reachability. There is expected to be significant implementation leeway.
9.10.3 Execution
At any time, if a set of objects and/or symbols S is not live, an ECMAScript
implementation may perform the following steps atomically:
1. For each element value of S, do
a. For each WeakRefref such that ref.[[WeakRefTarget]] is
value, do
i. Set ref.[[WeakRefTarget]] to empty.
b. For each FinalizationRegistryfg such that fg.[[Cells]] contains a Recordcell such that cell.[[WeakRefTarget]] is
value, do
c. For each WeakMap map such that map.[[WeakMapData]] contains a Recordr such that r.[[Key]] is value, do
i. Set r.[[Key]] to
empty.
ii. Set r.[[Value]] to
empty.
d. For each WeakSet set such that set.[[WeakSetData]] contains value, do
i. Replace the element of set.[[WeakSetData]] whose value is value with an element whose
value is empty.
Note 1
Together with the definition of liveness, this clause prescribes optimizations that an implementation
may apply regarding WeakRefs.
It is possible to access an object without observing its identity. Optimizations such as dead variable
elimination and scalar replacement on properties of non-escaping objects whose identity is not observed
are allowed. These optimizations are thus allowed to observably empty WeakRefs that point to
such objects.
On the other hand, if an object's identity is observable, and that object is in the [[WeakRefTarget]] internal slot of a WeakRef,
optimizations
such as rematerialization that observably empty the WeakRef
are prohibited.
Implementations are not obligated to empty WeakRefs for maximal
sets of non-live objects or symbols.
If an implementation chooses a non-live set S in which to
empty WeakRefs, this
definition requires that it empties WeakRefs for all values
in S simultaneously. In other words, it is not conformant for an implementation to empty a
WeakRef pointing to a
value v without emptying out other WeakRefs
that, if not
emptied, could result in an execution that observes the value of v.
The host-defined abstract operation
HostEnqueueFinalizationRegistryCleanupJob takes argument finalizationRegistry (a FinalizationRegistry)
and returns unused.
Let cleanupJob be a new JobAbstract
Closure with no parameters that captures finalizationRegistry and performs
the following steps when called:
An implementation of HostEnqueueFinalizationRegistryCleanupJob schedules cleanupJob to be
performed at some future time, if possible. It must also conform to the requirements in 9.5.
9.11 ClearKeptObjects ( )
The abstract operation ClearKeptObjects takes no arguments and returns unused.
ECMAScript implementations are expected to call ClearKeptObjects when a synchronous sequence of ECMAScript
executions completes. It performs the following steps when called:
When the abstract operation AddToKeptObjects is called with a target object or symbol, it adds the target to
a list that will point strongly at the target until ClearKeptObjects is called.
1. Assert: finalizationRegistry has
[[Cells]] and [[CleanupCallback]] internal slots.
2. Let callback be finalizationRegistry.[[CleanupCallback]].
3. While finalizationRegistry.[[Cells]] contains a Recordcell such that cell.[[WeakRefTarget]] is
empty, an implementation may perform the following steps:
a. Choose any such cell.
b. Remove cell from finalizationRegistry.[[Cells]].
c. Perform ? HostCallJobCallback(callback,
undefined, « cell.[[HeldValue]] »).
4. Return unused.
9.14 CanBeHeldWeakly ( v )
The abstract operation CanBeHeldWeakly takes argument v (an ECMAScript language
value) and returns a Boolean. It returns true if and only if
v is suitable for use as a weak reference. Only values that are suitable for use as a weak
reference may be a key of a WeakMap, an element of a WeakSet, the target of a WeakRef, or one of the
targets of a FinalizationRegistry.
It performs the following steps when called:
A language value without language identity can be manifested
without prior reference and is unsuitable for use as a weak reference. A Symbol value produced by
Symbol.for, unlike other Symbol
values, does not have language identity and is unsuitable for use as a weak reference. Well-known symbols are
likely to never be collected, but are nonetheless treated as suitable for use as a weak reference because
they are limited in number and therefore manageable by a variety of implementation approaches. However,
any value associated to a well-known symbol in a live WeakMap is unlikely to be
collected and could “leak” memory resources in implementations.
10 Ordinary and Exotic Objects Behaviours
10.1 Ordinary Object Internal Methods and Internal Slots
All ordinary objects have an internal slot
called [[Prototype]]. The value of this internal slot is either
null or an object and is used for implementing inheritance. Assume a property named
P is missing from an ordinary objectO but
exists on its [[Prototype]] object. If P refers to a data
property on the [[Prototype]] object, O inherits it for
get access, making it behave as if P was a property of O. If P refers to a
writable data property on the [[Prototype]] object, set access of P on O creates a new data
property named P on O. If P refers to a non-writable
data
property on the [[Prototype]] object, set access of P
on O fails. If P refers to an accessor property on the [[Prototype]] object, the accessor is inherited by O for both get access and
set access.
Every ordinary object has a Boolean-valued
[[Extensible]] internal slot which is used to fulfill the extensibility-related
internal method invariants specified in 6.1.7.3.
Namely, once the value of an object's [[Extensible]] internal slot has been set to
false, it is no longer possible to add properties to the object, to modify the value of the
object's [[Prototype]] internal slot, or to subsequently change the value of [[Extensible]] to true.
Each ordinary object internal method
delegates to a similarly-named abstract operation. If such an abstract operation depends on another internal
method, then the internal method is invoked on O rather than calling the similarly-named abstract
operation directly. These semantics ensure that exotic objects have their overridden
internal methods invoked when ordinary object internal methods are
applied to them.
10.1.1[[GetPrototypeOf]] ( )
The [[GetPrototypeOf]] internal method of an ordinary objectO takes no arguments and returns a normal
completion containing either an Object or null. It performs the
following steps when called:
The abstract operation OrdinaryGetPrototypeOf takes argument O (an Object) and returns an
Object or null. It performs the following steps when called:
1. Return O.[[Prototype]].
10.1.2[[SetPrototypeOf]] ( V )
The [[SetPrototypeOf]] internal method of an ordinary objectO takes argument V (an Object or null) and returns a normal completion
containing a Boolean. It performs the following steps when called:
The abstract operation OrdinarySetPrototypeOf takes arguments O (an Object) and V
(an Object or null) and returns a Boolean. It performs the following steps when called:
i. If p.[[GetPrototypeOf]] is not the ordinary
object internal method defined in 10.1.1,
set done to true.
ii. Else, set p to p.[[Prototype]].
8. Set O.[[Prototype]] to
V.
9. Return true.
Note
The loop in step 7 guarantees
that there will be no circularities in any prototype chain that only includes objects that use the
ordinary object definitions
for [[GetPrototypeOf]] and [[SetPrototypeOf]].
10.1.3[[IsExtensible]] ( )
The [[IsExtensible]] internal method of an ordinary objectO takes no arguments and returns a normal
completion containing a Boolean. It performs the following steps when called:
The abstract operation OrdinaryIsExtensible takes argument O (an Object) and returns a
Boolean. It performs the following steps when called:
1. Return O.[[Extensible]].
10.1.4[[PreventExtensions]] ( )
The [[PreventExtensions]] internal method of an ordinary objectO takes no arguments and returns a normal
completion containingtrue. It performs the following steps when
called:
The abstract operation OrdinaryGetOwnProperty takes arguments O (an Object) and P
(a property key) and returns a
Property
Descriptor or undefined. It performs the following steps when
called:
1. If O does not have an own property with key
P, return undefined.
10.1.6.2 IsCompatiblePropertyDescriptor ( Extensible,
Desc, Current )
The abstract operation IsCompatiblePropertyDescriptor takes arguments Extensible (a Boolean),
Desc (a Property
Descriptor), and Current (a Property
Descriptor or undefined) and returns a Boolean. It performs the
following steps when called:
10.1.6.3 ValidateAndApplyPropertyDescriptor ( O, P,
extensible, Desc, current )
The abstract operation ValidateAndApplyPropertyDescriptor takes arguments O (an Object or
undefined), P (a property key),
extensible (a Boolean), Desc (a Property
Descriptor), and current (a Property
Descriptor or undefined) and returns a Boolean. It returns
true if and only if Desc can be applied as the property of an object with
specified extensibility and current property current while upholding invariants.
When such application is possible and O is not undefined, it is performed
for the property named P (which is created if necessary). It performs the following steps when
called:
i. Create an own accessor
property named P of object O whose [[Get]], [[Set]], [[Enumerable]], and [[Configurable]] attributes
are set to the value of the corresponding field in Desc if Desc has that
field, or to the attribute's default
value otherwise.
d. Else,
i. Create an own data
property named P of object O whose [[Value]], [[Writable]], [[Enumerable]], and [[Configurable]] attributes
are set to the value of the corresponding field in Desc if Desc has that
field, or to the attribute's default
value otherwise.
i. If Desc has a [[Configurable]] field, let configurable be
Desc.[[Configurable]]; else let configurable be
current.[[Configurable]].
ii. If Desc has a [[Enumerable]] field, let enumerable be Desc.[[Enumerable]]; else let enumerable be
current.[[Enumerable]].
iii. Replace the property named P of object
O with an accessor property
whose [[Configurable]] and [[Enumerable]]
attributes are set to configurable and enumerable, respectively, and
whose [[Get]] and [[Set]] attributes are set
to the value of the corresponding field in Desc if Desc has that field,
or to the attribute's default
value otherwise.
i. If Desc has a [[Configurable]] field, let configurable be
Desc.[[Configurable]]; else let configurable be
current.[[Configurable]].
ii. If Desc has a [[Enumerable]] field, let enumerable be Desc.[[Enumerable]]; else let enumerable be
current.[[Enumerable]].
iii. Replace the property named P of object
O with a data property whose
[[Configurable]] and [[Enumerable]]
attributes are set to configurable and enumerable, respectively, and
whose [[Value]] and [[Writable]] attributes
are set to the value of the corresponding field in Desc if Desc has that
field, or to the attribute's default
value otherwise.
c. Else,
i. For each field of Desc, set the corresponding
attribute of the property named P of object O to the value of the field.
The abstract operation OrdinaryOwnPropertyKeys takes argument O (an Object) and returns a
List of
property keys. It performs the
following steps when called:
2. For each own property keyP of
O such that P is an array index, in ascending numeric
index order, do
a. Append P to keys.
3. For each own property keyP of
O such that Pis
a String and P is not an array index, in ascending
chronological order of property creation, do
a. Append P to keys.
4. For each own property keyP of
O such that Pis
a Symbol, in ascending chronological order of property creation, do
a. Append P to keys.
5. Return keys.
10.1.12 OrdinaryObjectCreate ( proto [ ,
additionalInternalSlotsList ] )
The abstract operation OrdinaryObjectCreate takes argument proto (an Object or
null) and optional argument additionalInternalSlotsList (a List of names
of internal slots) and returns an Object. It is used to specify the runtime creation of new ordinary
objects. additionalInternalSlotsList contains the names of additional internal
slots that must be defined as part of the object, beyond [[Prototype]] and [[Extensible]]. If additionalInternalSlotsList is not provided, a new empty
List is used.
It performs the following steps when called:
1. Let internalSlotsList be « [[Prototype]], [[Extensible]] ».
2. If additionalInternalSlotsList is present, set
internalSlotsList to the list-concatenation of
internalSlotsList and additionalInternalSlotsList.
Although OrdinaryObjectCreate does little more than call MakeBasicObject,
its use
communicates the intention to create an ordinary object, and not an
exotic one. Thus, within this specification, it is not called by any algorithm that subsequently
modifies the internal methods of the object in ways that would make the result non-ordinary. Operations
that create exotic objects invoke MakeBasicObject directly.
The abstract operation OrdinaryCreateFromConstructor takes arguments constructor (a constructor) and
intrinsicDefaultProto (a String) and optional argument internalSlotsList (a List of names
of internal slots) and returns either a normal
completion containing an Object or a throw
completion. It creates an ordinary object whose [[Prototype]] value is retrieved from a constructor's
"prototype" property, if it exists. Otherwise the intrinsic named by
intrinsicDefaultProto is used for [[Prototype]].
internalSlotsList contains the names of additional internal slots that must be defined as part of
the object. If internalSlotsList is not provided, a new empty List is used.
It performs the following steps when called:
1. Assert: intrinsicDefaultProto
is this specification's name of an intrinsic object. The corresponding object must be an intrinsic that
is intended to be used as the [[Prototype]] value of an object.
The abstract operation GetPrototypeFromConstructor takes arguments constructor (a function
object) and intrinsicDefaultProto (a String) and returns either a normal completion
containing an Object or a throw
completion. It determines the [[Prototype]] value that should be
used to create an object corresponding to a specific constructor. The value is retrieved from
the constructor's
"prototype" property, if it exists. Otherwise the intrinsic named by
intrinsicDefaultProto is used for [[Prototype]]. It performs the
following steps when called:
1. Assert: intrinsicDefaultProto
is this specification's name of an intrinsic object. The corresponding object must be an intrinsic that
is intended to be used as the [[Prototype]] value of an object.
b. Set proto to realm's intrinsic object
named intrinsicDefaultProto.
4. Return proto.
Note
If constructor does not supply a [[Prototype]] value, the default
value that is used is obtained from the realm of the constructor
function rather than from the running execution
context.
10.1.15 RequireInternalSlot ( O, internalSlot )
The abstract operation RequireInternalSlot takes arguments O (an ECMAScript language
value) and internalSlot (an internal slot name) and returns either a normal completion
containingunused or a throw
completion. It throws an exception unless Ois an Object and has
the given internal slot. It performs the following steps when called:
The PrivateEnvironment
Record for Private Names that the
function was closed over. null if this function is not syntactically contained
within a class. Used as the outer PrivateEnvironment for inner classes when evaluating the code of
the function.
The script or module in which the function was created.
[[ThisMode]]
lexical, strict, or global
Defines how this references are interpreted within the formal parameters and code body
of the function. lexical means that this refers to the
this value of a lexically enclosing function. strict means
that the this value is used exactly as provided by an invocation of the function.
global means that a this value of
undefined or null is interpreted as a reference to the
global object, and any
other this value is first passed to ToObject.
If the function is created as the initializer of a class field, the name to use for NamedEvaluation
of the field; empty otherwise.
[[IsClassConstructor]]
a Boolean
Indicates whether the function is a class constructor. (If
true, invoking the function's [[Call]] will immediately
throw a TypeError exception.)
All ECMAScript function objects have the [[Call]] internal method defined here. ECMAScript functions that are also constructors in addition have the [[Construct]] internal method.
When calleeContext is removed from the execution context
stack in step 7 it must not be
destroyed if it is suspended and retained for later resumption by an accessible Generator.
10.2.1.1 PrepareForOrdinaryCall ( F, newTarget )
The abstract operation PrepareForOrdinaryCall takes arguments F (an ECMAScript function
object) and newTarget (an Object or undefined) and
returns an execution context. It
performs the following steps when called:
The abstract operation OrdinaryCallBindThis takes arguments F (an ECMAScript function
object), calleeContext (an execution
context), and thisArgument (an ECMAScript language
value) and returns unused. It performs the following steps when
called:
1. Let thisMode be F.[[ThisMode]].
2. If thisMode is lexical, return
unused.
3. Let calleeRealm be F.[[Realm]].
4. Let localEnv be the LexicalEnvironment of
calleeContext.
5. If thisMode is strict, then
a. Let thisValue be thisArgument.
6. Else,
a. If thisArgument is either
undefined or null, then
Even though field initializers constitute a function boundary, calling FunctionDeclarationInstantiation
does not have any observable effect and so is omitted.
The abstract operation OrdinaryFunctionCreate takes arguments functionPrototype (an Object),
sourceText (a sequence of Unicode code points), ParameterList (a Parse Node), Body
(a Parse Node),
thisMode (lexical-this or non-lexical-this),
env (an Environment Record), and
privateEnv (a PrivateEnvironment Record
or null) and returns an ECMAScript function object. It is used to
specify the runtime creation of a new function with a default [[Call]] internal
method and no [[Construct]] internal method (although one may be subsequently added
by an operation such as MakeConstructor).
sourceText is the source text of the syntactic definition of the function to be created. It
performs the following steps when called:
1. Let internalSlotsList be the internal slots listed in
Table
30.
The abstract operation AddRestrictedFunctionProperties takes arguments F (a function
object) and realm (a Realm Record) and returns
unused. It performs the following steps when called:
The abstract operation MakeConstructor takes argument F (an ECMAScript function
object or a built-in function object) and optional
arguments writablePrototype (a Boolean) and prototype (an Object) and returns
unused. It converts F into a constructor. It performs
the following steps when called:
The abstract operation MakeClassConstructor takes argument F (an ECMAScript function
object) and returns unused. It performs the following steps when
called:
The abstract operation MakeMethod takes arguments F (an ECMAScript function
object) and homeObject (an Object) and returns unused.
It configures F as a method. It performs the following steps when called:
The abstract operation SetFunctionName takes arguments F (a function object) and
name (a property key or Private
Name) and optional argument prefix (a String) and returns
unused. It adds a "name" property to F. It performs the
following steps when called:
1. Assert: F is an extensible
object that does not have a "name" own property.
The abstract operation SetFunctionLength takes arguments F (a function object) and
length (a non-negative integer or +∞) and returns
unused. It adds a "length" property to F. It performs
the following steps when called:
1. Assert: F is an extensible
object that does not have a "length" own property.
When an execution context is
established for evaluating an ECMAScript function a new Function Environment
Record is created and bindings for each formal parameter are instantiated in that
Environment Record. Each
declaration in the function body is also instantiated. If the function's formal parameters do not
include any default value initializers then the body declarations are instantiated in the same Environment Record as
the parameters. If default value parameter initializers exist, a second Environment Record is
created for the body declarations. Formal parameters and functions are initialized as part of
FunctionDeclarationInstantiation. All other bindings are initialized during evaluation of the function
body.
ii. Let fn be the sole element of the BoundNames
of d.
iii. If functionNames does not contain
fn, then
1. Insert fn as the first element of
functionNames.
2. NOTE: If there are multiple function declarations for
the same name, the last declaration is used.
3. Insert d as the first element of
functionsToInitialize.
15. Let argumentsObjectNeeded be true.
16. If func.[[ThisMode]] is
lexical, then
a. NOTE: Arrow functions never have an arguments object.
b. Set argumentsObjectNeeded to
false.
17. Else if parameterNames contains
"arguments", then
a. Set argumentsObjectNeeded to
false.
18. Else if hasParameterExpressions is
false, then
a. If functionNames contains
"arguments" or lexicalNames contains "arguments",
then
i. Set argumentsObjectNeeded to
false.
19. If strict is true or
hasParameterExpressions is false, then
a. NOTE: Only a single Environment
Record is needed for the parameters, since calls to eval in
strict mode code cannot
create new bindings which are visible outside of the eval.
b. Let env be the LexicalEnvironment of
calleeContext.
20. Else,
a. NOTE: A separate Environment
Record is needed to ensure that bindings created by direct
eval calls in the formal parameter list are outside the environment where
parameters are declared.
b. Let calleeEnv be the LexicalEnvironment of
calleeContext.
d. Assert: The VariableEnvironment of
calleeContext is calleeEnv.
e. Set the LexicalEnvironment of calleeContext to
env.
21. For each String paramName of parameterNames,
do
a. Let alreadyDeclared be
! env.HasBinding(paramName).
b. NOTE: Early errors ensure that
duplicate parameter names can only occur in non-strict functions
that do not have parameter default values or rest parameters.
c. If alreadyDeclared is false, then
i. Perform
! env.CreateMutableBinding(paramName, false).
i. NOTE: A mapped argument object is only provided for
non-strict functions
that don't have a rest parameter, any parameter default value initializers, or any destructured
parameters.
a. NOTE: Only a single Environment
Record is needed for the parameters and top-level vars.
b. Let instantiatedVarNames be a copy of the ListparameterBindings.
c. For each element n of varNames, do
i. If instantiatedVarNames does not contain
n, then
1. Append n to instantiatedVarNames.
2. Perform
! env.CreateMutableBinding(n, false).
3. Perform
! env.InitializeBinding(n, undefined).
d. Let varEnv be env.
28. Else,
a. NOTE: A separate Environment
Record is needed to ensure that closures created by expressions in the formal
parameter list do not have visibility of declarations in the function body.
b. NOTE: Non-strict functions use
a separate Environment Record
for top-level lexical declarations so that a direct
eval can determine whether any var scoped declarations introduced by the eval
code conflict with pre-existing top-level lexically scoped declarations. This is not needed for
strict functions because a
strict direct
eval always places all declarations into a new Environment Record.
31. Else,
a. Let lexEnv be varEnv.
32. Set the LexicalEnvironment of calleeContext to
lexEnv.
a. NOTE: A lexically declared name cannot be the same as a
function/generator declaration, formal parameter, or a var name. Lexically declared names are only
instantiated here but not initialized.
c. Perform
! varEnv.SetMutableBinding(fn, fo, false).
37. Return unused.
Note 2
B.3.2
provides an extension to the above algorithm that is necessary for backwards compatibility with web
browser implementations of ECMAScript that predate ECMAScript 2015.
In addition to the internal slots required of every ordinary object (see 10.1),
a built-in function object must also have the
following internal slots:
[[Realm]], a Realm Record that represents the
realm in
which the function was created.
[[InitialName]], a String that is the initial name of the function. It is used by
20.2.3.5.
A built-in function object must have a [[Call]] internal method that conforms to the definition in 10.3.1.
A built-in function object has a [[Construct]] internal method if and only if it is described as a “constructor”, or some algorithm in this
specification explicitly sets its [[Construct]] internal method. Such a [[Construct]] internal method must conform to the definition in 10.3.2.
An implementation may provide additional built-in function objects that are not defined
in this specification.
10. Let result be
the Completion
Record that is the result of evaluatingF
in a manner that conforms to the specification of F. If thisArgument is
uninitialized, the this value is uninitialized; otherwise,
thisArgument provides the this value. argumentsList provides
the named parameters. newTarget provides the NewTarget value.
11. NOTE: If F is defined in this document, “the
specification of F” is the behaviour specified for it via algorithm steps or other means.
When calleeContext is removed from the execution context
stack it must not be destroyed if it has been suspended and retained by an accessible
Generator for later resumption.
The abstract operation CreateBuiltinFunction takes arguments behaviour (an Abstract Closure, a set of
algorithm steps, or some other definition of a function's behaviour provided in this specification),
length (a non-negative integer or +∞), name (a property
key or a Private Name), and
additionalInternalSlotsList (a List of names
of internal slots) and optional arguments realm (a Realm Record),
prototype (an Object or null), and prefix (a String) and returns a
function object.
additionalInternalSlotsList contains the names of additional internal slots that must be defined
as part of the object. This operation creates a built-in function object. It performs the
following steps when called:
2. If prototype is not present, set prototype to
realm.[[Intrinsics]].[[%Function.prototype%]].
3. Let internalSlotsList be a List
containing the names of all the internal slots that 10.3
requires for
the built-in function object that is about to
be created.
4. Append to internalSlotsList the elements of
additionalInternalSlotsList.
5. Let func be a new built-in function
object that, when
called, performs the action described by behaviour using the provided arguments as the values
of the corresponding parameters specified by behaviour. The new function
object has internal
slots whose names are the elements of internalSlotsList, and an [[InitialName]] internal slot.
Each built-in function defined in this specification is created by calling the CreateBuiltinFunction
abstract operation.
10.4 Built-in Exotic Object Internal Methods and Slots
This specification defines several kinds of built-in exotic objects. These objects generally
behave similar to ordinary objects except for a few
specific situations. The following exotic objects use the ordinary
object internal methods except where it is explicitly specified otherwise below:
An object is a bound function exotic object if its [[Call]] and (if
applicable) [[Construct]] internal methods use the following implementations, and
its other essential internal methods use the definitions found in 10.1.
These methods are installed in BoundFunctionCreate.
a. Set obj.[[Construct]] as
described in 10.4.1.2.
7. Set obj.[[BoundTargetFunction]] to targetFunction.
8. Set obj.[[BoundThis]] to
boundThis.
9. Set obj.[[BoundArguments]] to
boundArgs.
10. Return obj.
10.4.2 Array Exotic Objects
An Array is an exotic object that gives special
treatment to array indexproperty
keys (see 6.1.7). A property whose property
name is an array index is also called an
element. Every Array has a non-configurable "length" property whose value is
always a non-negative integral Number whose mathematical value is strictly
less than 2**32. The value of the "length"
property is numerically greater than the name of every own property whose name is an array
index; whenever an own property of an Array is created or changed, other properties are
adjusted as necessary to maintain this invariant. Specifically, whenever an own property is added whose name
is an array
index, the value of the "length" property is changed, if necessary, to
be one more than the numeric value of that array index; and whenever the value of
the "length" property is changed, every own property whose name is an array
index whose value is not smaller than the new length is deleted. This constraint applies
only to own properties of an Array and is unaffected by "length" or array
index properties that may be inherited from its prototypes.
An object is an Array exotic
object (or simply, an Array) if its [[DefineOwnProperty]] internal method
uses the following implementation, and its other essential internal methods use the definitions found in
10.1.
These methods are installed in ArrayCreate.
The abstract operation ArrayCreate takes argument length (a non-negative integer)
and optional argument proto (an Object) and returns either a normal completion
containing an Array exotic object or a
throw
completion. It is used to specify the creation of new Arrays. It performs the following
steps when called:
1. If length > 2**32 - 1, throw a RangeError exception.
The abstract operation ArraySpeciesCreate takes arguments originalArray (an Object) and
length (a non-negative integer) and returns either a normal completion
containing an Object or a throw
completion. It is used to specify the creation of a new Array or similar object using a
constructor function that is derived
from originalArray. It does not enforce that the constructor function
returns an Array. It performs the following steps when called:
If originalArray was created using the standard built-in Array constructor for a realm that
is not the realm of the running execution
context, then a new Array is created using the realm of the running execution
context. This maintains compatibility with Web browsers that have historically had
that behaviour for the Array.prototype methods that now are defined using
ArraySpeciesCreate.
In steps 3 and 4, if
Desc.[[Value]] is an object then its valueOf method
is called twice. This is legacy behaviour that was specified with this effect starting with the
2nd Edition of this specification.
10.4.3 String Exotic Objects
A String object is an exotic object that encapsulates a
String value and exposes virtual integer-indexeddata
properties corresponding to the individual code unit elements of the String value.
String exotic objects always
have a data property named
"length" whose value is the length of the encapsulated String value. Both the code unit
data
properties and the
"length" property are non-writable and non-configurable.
An object is a String exotic
object (or simply, a String object) if its [[GetOwnProperty]], [[DefineOwnProperty]], and [[OwnPropertyKeys]] internal
methods use the following implementations, and its other essential internal methods use the definitions
found in 10.1.
These methods are installed in StringCreate.
7. For each own property keyP of
O such that Pis
a String and P is not an array index, in ascending
chronological order of property creation, do
a. Append P to keys.
8. For each own property keyP of
O such that Pis
a Symbol, in ascending chronological order of property creation, do
a. Append P to keys.
9. Return keys.
10.4.3.4 StringCreate ( value, prototype )
The abstract operation StringCreate takes arguments value (a String) and prototype
(an Object) and returns a String exotic object. It is
used to specify the creation of new String exotic objects. It
performs the following steps when called:
1. Let S be MakeBasicObject(« [[Prototype]], [[Extensible]], [[StringData]] »).
2. Set S.[[Prototype]] to
prototype.
3. Set S.[[StringData]] to
value.
4. Set S.[[GetOwnProperty]] as
specified in 10.4.3.1.
5. Set S.[[DefineOwnProperty]]
as specified in 10.4.3.2.
6. Set S.[[OwnPropertyKeys]] as
specified in 10.4.3.3.
The abstract operation StringGetOwnProperty takes arguments S (an Object that has a [[StringData]] internal slot) and P (a property key) and
returns a Property
Descriptor or undefined. It performs the following steps when
called:
Most ECMAScript functions make an arguments object available to their code. Depending upon the
characteristics of the function definition, its arguments object is either an ordinary
object or an arguments exotic object. An
arguments exotic object is
an exotic
object whose array
index properties map to the formal parameters bindings of an invocation of its associated
ECMAScript function.
An object is an arguments exotic object if its internal methods use the following implementations,
with the ones not specified here using those found in 10.1.
These methods are installed in CreateMappedArgumentsObject.
Arguments exotic objects
have the same internal slots as ordinary objects. They also have a
[[ParameterMap]] internal slot. Ordinary arguments objects also have a [[ParameterMap]] internal slot whose value is always undefined. For ordinary argument
objects the [[ParameterMap]] internal slot is only used by
Object.prototype.toString (20.1.3.6) to identify
them as such.
Note 2
The integer-indexeddata
properties of an arguments exotic object
whose numeric name values are less than the number of formal parameters of the corresponding function
object initially share
their values with the corresponding argument bindings in the function's execution context. This
means that changing the property changes the corresponding value of the argument binding and vice-versa.
This correspondence is broken if such a property is deleted and then redefined or if the property is
changed into an accessor property. If the
arguments object is an ordinary object, the values of
its properties are simply a copy of the arguments passed to the function and there is no dynamic linkage
between the property values and the formal parameter values.
Note 3
The ParameterMap object and its property values are used as a device for specifying the arguments
object correspondence to argument bindings. The ParameterMap object and the objects that are the values
of its properties are not directly observable from ECMAScript code. An ECMAScript implementation does
not need to actually create or use such objects to implement the specified semantics.
Note 4
Ordinary arguments objects define a non-configurable accessor
property named "callee" which throws a
TypeError exception on access. The "callee" property has a more
specific meaning for arguments exotic
objects, which are created only for some class of non-strict functions. The
definition of this property in the ordinary variant exists to ensure that it is not defined in any other
manner by conforming ECMAScript implementations.
Note 5
ECMAScript implementations of arguments exotic objects
have historically contained an accessor property named
"caller". Prior to ECMAScript 2017, this specification included the definition of a
throwing "caller" property on ordinary arguments objects. Since implementations do
not contain this extension any longer, ECMAScript 2017 dropped the requirement for a throwing
"caller" accessor.
The abstract operation CreateUnmappedArgumentsObject takes argument argumentsList (a List of
ECMAScript language
values) and returns an ordinary object. It performs the
following steps when called:
1. Let len be the number of elements in
argumentsList.
The abstract operation MakeArgGetter takes arguments name (a String) and env (an
Environment Record) and
returns a function object. It creates a
built-in function object that when
executed returns the value bound for name in env. It performs the following steps
when called:
1. Let getterClosure be a new Abstract Closure with
no parameters that captures name and env and performs the following steps when
called:
3. NOTE: getter is never directly accessible to
ECMAScript code.
4. Return getter.
10.4.4.7.2 MakeArgSetter ( name, env )
The abstract operation MakeArgSetter takes arguments name (a String) and env (an
Environment Record) and
returns a function object. It creates a
built-in function object that when
executed sets the value bound for name in env. It performs the following steps
when called:
1. Let setterClosure be a new Abstract Closure with
parameters (value) that captures name and env and performs the
following steps when called:
a. Return
! env.SetMutableBinding(name, value,
false).
TypedArrays have the same internal slots
as ordinary objects and additionally
[[ViewedArrayBuffer]], [[ArrayLength]], [[ByteOffset]], [[ContentType]], and [[TypedArrayName]] internal slots.
An object is a TypedArray if its [[GetOwnProperty]], [[HasProperty]], [[DefineOwnProperty]], [[Get]], [[Set]], [[Delete]], and [[OwnPropertyKeys]] internal methods use the definitions in this section, and its
other essential internal methods use the definitions found in 10.1.
These methods are installed by TypedArrayCreate.
4. For each own property keyP of
O such that Pis
a String and P is not an integer index,
in ascending chronological order of property creation, do
a. Append P to keys.
5. For each own property keyP of
O such that Pis
a Symbol, in ascending chronological order of property creation, do
a. Append P to keys.
6. Return keys.
10.4.5.8 TypedArray With Buffer Witness Records
An TypedArray With Buffer Witness
Record is a Record
value used to encapsulate a TypedArray along with a cached byte
length of the viewed buffer. It is used to help ensure there is a single shared memory read event of the
byte length data block when the viewed buffer is a growable
SharedArrayBuffer.
TypedArray With Buffer Witness Records have the fields listed in Table
32.
The byte length of the object's [[ViewedArrayBuffer]] when the
Record
was created.
10.4.5.9 MakeTypedArrayWithBufferWitnessRecord ( obj,
order )
The abstract operation MakeTypedArrayWithBufferWitnessRecord takes arguments obj (a TypedArray) and order
(seq-cst or unordered) and returns a TypedArray With Buffer
Witness Record. It performs the following steps when called:
The abstract operation TypedArrayCreate takes argument prototype (an Object) and returns a
TypedArray. It is used to specify the
creation of new TypedArrays. It performs the following
steps when called:
1. Let internalSlotsList be « [[Prototype]], [[Extensible]], [[ViewedArrayBuffer]], [[TypedArrayName]], [[ContentType]], [[ByteLength]], [[ByteOffset]], [[ArrayLength]] ».
9. Set A.[[OwnPropertyKeys]] as
specified in 10.4.5.7.
10. Set A.[[Prototype]] to
prototype.
11. Return A.
10.4.5.11 TypedArrayByteLength ( taRecord )
The abstract operation TypedArrayByteLength takes argument taRecord (a TypedArray With Buffer
Witness Record) and returns a non-negative integer. It performs the following steps
when called:
The abstract operation TypedArrayLength takes argument taRecord (a TypedArray With Buffer
Witness Record) and returns a non-negative integer. It performs the following steps
when called:
The abstract operation IsTypedArrayOutOfBounds takes argument taRecord (a TypedArray With Buffer
Witness Record) and returns a Boolean. It checks if any of the object's numeric
properties reference a value at an index not contained within the underlying buffer's bounds. It performs
the following steps when called:
1. Let O be taRecord.[[Object]].
2. Let bufferByteLength be taRecord.[[CachedBufferByteLength]].
3. Assert: IsDetachedBuffer(O.[[ViewedArrayBuffer]]) is true if and only if
bufferByteLength is detached.
b. Let byteOffsetEnd be byteOffsetStart +
O.[[ArrayLength]] × elementSize.
8. If byteOffsetStart > bufferByteLength or
byteOffsetEnd > bufferByteLength, return true.
9. NOTE: 0-length TypedArrays are not considered
out-of-bounds.
10. Return false.
10.4.5.14 IsValidIntegerIndex ( O, index )
The abstract operation IsValidIntegerIndex takes arguments O (a TypedArray) and
index (a Number) and returns a Boolean. It performs the following steps when called:
1. If IsDetachedBuffer(O.[[ViewedArrayBuffer]]) is true, return
false.
8. If ℝ(index) < 0 or
ℝ(index) ≥
length, return false.
9. Return true.
10.4.5.15 TypedArrayGetElement ( O, index )
The abstract operation TypedArrayGetElement takes arguments O (a TypedArray) and
index (a Number) and returns a Number, a BigInt, or undefined. It performs
the following steps when called:
e. Perform SetValueInBuffer(O.[[ViewedArrayBuffer]], byteIndexInBuffer, elementType,
numValue, true, unordered).
4. Return unused.
Note
This operation always appears to succeed, but it has no effect when attempting to write past the end
of a TypedArray or to a TypedArray which is backed by a
detached ArrayBuffer.
10.4.5.17 IsArrayBufferViewOutOfBounds ( O )
The abstract operation IsArrayBufferViewOutOfBounds takes argument O (a TypedArray or a DataView) and returns a
Boolean. It checks if either any of a TypedArray's numeric properties or a
DataView object's methods can reference a value at an index not contained within the underlying data
block's bounds. This abstract operation exists as a convenience for upstream specifications. It performs
the following steps when called:
A module namespace exotic
object is an exotic object that exposes the
bindings exported from an ECMAScript Module (See 16.2.3). There is a
one-to-one correspondence between the String-keyed own properties of a module namespace exotic
object and the binding names exported by the Module. The exported bindings include any
bindings that are indirectly exported using export * export items. Each String-valued own
property key is the StringValue of the
corresponding exported binding name. These are the only String-keyed properties of a module namespace exotic
object. Each such property has the attributes { [[Writable]]:
true, [[Enumerable]]: true, [[Configurable]]: false }. Module namespace exotic
objects are not extensible.
An object is a module namespace exotic object if its [[GetPrototypeOf]],
[[SetPrototypeOf]], [[IsExtensible]], [[PreventExtensions]], [[GetOwnProperty]], [[DefineOwnProperty]], [[HasProperty]], [[Get]], [[Set]], [[Delete]], and
[[OwnPropertyKeys]] internal methods use the definitions in this section, and its
other essential internal methods use the definitions found in 10.1.
These methods are installed by ModuleNamespaceCreate.
A List
whose elements are the String values of the exported names exposed as own properties of this
object. The list is ordered as if an Array of those String values had been sorted using
%Array.prototype.sort% using undefined as comparefn.
ResolveExport is side-effect free. Each time this operation is called with a specific
exportName, resolveSet pair as arguments it must return the same result. An
implementation might choose to pre-compute or cache the ResolveExport results for the [[Exports]] of each module
namespace exotic object.
4. Set M's essential internal methods to the definitions
specified in 10.4.6.
5. Set M.[[Module]] to
module.
6. Let sortedExports be a List
whose elements are the elements of exports ordered as if an Array of the same values had
been sorted using %Array.prototype.sort% using undefined as comparefn.
7. Set M.[[Exports]] to
sortedExports.
8. Create own properties of M corresponding to the
definitions in 28.3.
An object is an immutable prototype exotic object if its [[SetPrototypeOf]]
internal method uses the following implementation. (Its other essential internal methods may use any
implementation, depending on the specific immutable
prototype exotic object in question.)
The abstract operation SetImmutablePrototype takes arguments O (an Object) and V
(an Object or null) and returns either a normal completion
containing a Boolean or a throw
completion. It performs the following steps when called:
10.5 Proxy Object Internal Methods and Internal Slots
A Proxy object is an exotic object whose essential internal
methods are partially implemented using ECMAScript code. Every Proxy object has an internal slot called [[ProxyHandler]]. The value of [[ProxyHandler]] is an object,
called the proxy's handler object, or null. Methods (see Table 34) of a handler
object may be used to augment the implementation for one or more of the Proxy object's internal methods. Every
Proxy object also has an internal slot called [[ProxyTarget]] whose value is either
an object or the null value. This object is called the proxy's target object.
An object is a Proxy exotic
object if its essential internal methods (including [[Call]] and [[Construct]], if applicable) use the definitions in this section. These internal
methods are installed in ProxyCreate.
Table 34: Proxy Handler Methods
Internal Method
Handler Method
[[GetPrototypeOf]]
getPrototypeOf
[[SetPrototypeOf]]
setPrototypeOf
[[IsExtensible]]
isExtensible
[[PreventExtensions]]
preventExtensions
[[GetOwnProperty]]
getOwnPropertyDescriptor
[[DefineOwnProperty]]
defineProperty
[[HasProperty]]
has
[[Get]]
get
[[Set]]
set
[[Delete]]
deleteProperty
[[OwnPropertyKeys]]
ownKeys
[[Call]]
apply
[[Construct]]
construct
When a handler method is called to provide the implementation of a Proxy object internal method, the handler
method is passed the proxy's target object as a parameter. A proxy's handler object does not necessarily have
a method corresponding to every essential internal method. Invoking an internal method on the proxy results in
the invocation of the corresponding internal method on the proxy's target object if the handler object does
not have a method corresponding to the internal trap.
The [[ProxyHandler]] and [[ProxyTarget]] internal slots of
a Proxy object are always initialized when the object is created and typically may not be modified. Some Proxy
objects are created in a manner that permits them to be subsequently revoked. When a proxy is
revoked, its [[ProxyHandler]] and [[ProxyTarget]] internal
slots are set to null causing subsequent invocations of internal methods on that Proxy
object to throw a TypeError exception.
Because Proxy objects permit the implementation of internal methods to be provided by arbitrary ECMAScript
code, it is possible to define a Proxy object whose handler methods violates the invariants defined in
6.1.7.3.
Some of the internal method invariants defined in 6.1.7.3
are essential integrity invariants. These invariants are explicitly enforced by the Proxy object internal
methods specified in this section. An ECMAScript implementation must be robust in the presence of all possible
invariant violations.
5. Let trap be ? GetMethod(handler,
"getPrototypeOf").
6. If trap is undefined, then
a. Return ? target.[[GetPrototypeOf]]().
7. Let handlerProto be ? Call(trap, handler, «
target »).
8. If handlerProtois not an Object
and handlerProto is not null, throw a TypeError
exception.
9. Let extensibleTarget be ? IsExtensible(target).
10. If extensibleTarget is true, return
handlerProto.
11. Let targetProto be ? target.[[GetPrototypeOf]]().
12. If SameValue(handlerProto,
targetProto) is false, throw a TypeError exception.
13. Return handlerProto.
Note
[[GetPrototypeOf]] for Proxy objects enforces the following invariants:
The result of [[GetPrototypeOf]] must be either an Object or
null.
If the target object is not extensible, [[GetPrototypeOf]] applied to the
Proxy object must return the same value as [[GetPrototypeOf]] applied to the
Proxy object's target object.
[[IsExtensible]] applied to the Proxy object must return the same value as
[[IsExtensible]] applied to the Proxy object's target object with the same
argument.
ii. If targetDesc.[[Writable]] is true, throw a
TypeError exception.
17. Return resultDesc.
Note
[[GetOwnProperty]] for Proxy objects enforces the following invariants:
The result of [[GetOwnProperty]] must be either an Object or
undefined.
A property cannot be reported as non-existent, if it exists as a non-configurable own property of the
target object.
A property cannot be reported as non-existent, if it exists as an own property of a non-extensible
target object.
A property cannot be reported as existent, if it does not exist as an own property of the target
object and the target object is not extensible.
A property cannot be reported as non-configurable, unless it exists as a non-configurable own property
of the target object.
A property cannot be reported as both non-configurable and non-writable, unless it exists as a
non-configurable, non-writable own property of the target object.
b. If settingConfigFalse is true and
targetDesc.[[Configurable]] is true, throw a
TypeError exception.
c. If IsDataDescriptor(targetDesc)
is true, targetDesc.[[Configurable]] is
false, and targetDesc.[[Writable]] is
true, then
i. If Desc has a [[Writable]] field and Desc.[[Writable]] is false, throw a
TypeError exception.
16. Return true.
Note
[[DefineOwnProperty]] for Proxy objects enforces the following invariants:
The result of [[DefineOwnProperty]]is a
Boolean value.
A property cannot be added, if the target object is not extensible.
A property cannot be non-configurable, unless there exists a corresponding non-configurable own
property of the target object.
A non-configurable property cannot be non-writable, unless there exists a corresponding
non-configurable, non-writable own property of the target object.
If a property has a corresponding target object property then applying the Property
Descriptor of the property to the target object using [[DefineOwnProperty]] will not throw an exception.
7. Let trapResult be ? Call(trap, handler, « target,
P, Receiver »).
8. Let targetDesc be ? target.[[GetOwnProperty]](P).
9. If targetDesc is not undefined and
targetDesc.[[Configurable]] is false, then
a. If IsDataDescriptor(targetDesc)
is true and targetDesc.[[Writable]] is
false, then
i. If SameValue(trapResult,
targetDesc.[[Value]]) is false, throw a
TypeError exception.
b. If IsAccessorDescriptor(targetDesc)
is true and targetDesc.[[Get]] is
undefined, then
i. If trapResult is not
undefined, throw a TypeError exception.
10. Return trapResult.
Note
[[Get]] for Proxy objects enforces the following invariants:
The value reported for a property must be the same as the value of the corresponding target object
property if the target object property is a non-writable, non-configurable own data
property.
The value reported for a property must be undefined if the corresponding target
object property is a non-configurable own accessor property that has
undefined as its [[Get]] attribute.
Cannot change the value of a property to be different from the value of the corresponding target
object property if the corresponding target object property is a non-writable, non-configurable own
data property.
Cannot set the value of a property if the corresponding target object property is a non-configurable
own accessor property that has
undefined as its [[Set]] attribute.
A Proxy exotic object only has
a [[Call]] internal method if the initial value of its [[ProxyTarget]] internal slot is an object that has a [[Call]] internal method.
A Proxy exotic object only has
a [[Construct]] internal method if the initial value of its [[ProxyTarget]] internal slot is an object that has a [[Construct]] internal method.
Note 2
[[Construct]] for Proxy objects enforces the following invariants:
The result of [[Construct]] must be an Object.
10.5.14 ValidateNonRevokedProxy ( proxy )
The abstract operation ValidateNonRevokedProxy takes argument proxy (a Proxy
exotic object) and returns
either a normal completion
containingunused or a throw
completion. It throws a TypeError exception if proxy has
been revoked. It performs the following steps when called:
1. If proxy.[[ProxyTarget]] is
null, throw a TypeError exception.
ECMAScript source text is a sequence of Unicode code points. All Unicode code point
values from U+0000 to U+10FFFF, including surrogate code points, may occur in ECMAScript source text where
permitted by the ECMAScript grammars. The actual encodings used to store and interchange ECMAScript source
text is not relevant to this specification. Regardless of the external source text encoding, a conforming
ECMAScript implementation processes the source text as if it was an equivalent sequence of SourceCharacter
values, each SourceCharacter being a Unicode
code point. Conforming ECMAScript implementations are not required to perform any normalization of source
text, or behave as though they were performing normalization of source text.
The components of a combining character sequence are treated as individual Unicode code points even though a
user might think of the whole sequence as a single character.
Note
In string literals, regular expression literals, template literals and identifiers, any Unicode code
point may also be expressed using Unicode escape sequences that explicitly express a code point's numeric
value. Within a comment, such an escape sequence is effectively ignored as part of the comment.
ECMAScript differs from the Java programming language in the behaviour of Unicode escape sequences. In a
Java program, if the Unicode escape sequence \u000A, for example, occurs within a single-line
comment, it is interpreted as a line terminator (Unicode code point U+000A is LINE FEED (LF)) and
therefore the next code point is not part of the comment. Similarly, if the Unicode escape sequence
\u000A occurs within a string literal in a Java program, it is likewise interpreted as a line
terminator, which is not allowed within a string literal—one must write \n instead of
\u000A to cause a LINE FEED (LF) to be part of the String value of a string literal. In an
ECMAScript program, a Unicode escape sequence occurring within a comment is never interpreted and
therefore cannot contribute to termination of the comment. Similarly, a Unicode escape sequence occurring
within a string literal in an ECMAScript program always contributes to the literal and is never
interpreted as a line terminator or as a code point that might terminate the string literal.
The abstract operation UTF16EncodeCodePoint takes argument cp (a Unicode code point) and returns
a String. It performs the following steps when called:
11.1.2 Static Semantics: CodePointsToString ( text )
The abstract operation CodePointsToString takes argument text (a sequence of Unicode code
points) and returns a String. It converts text into a String value, as described in 6.1.4. It
performs the following steps when called:
The abstract operation UTF16SurrogatePairToCodePoint takes arguments lead (a code unit) and
trail (a code unit) and returns a code point. Two code units that form a UTF-16 surrogate
pair are converted to a code point. It performs the following steps when called:
2. Let cp be (lead - 0xD800) × 0x400 +
(trail - 0xDC00) + 0x10000.
3. Return the code point cp.
11.1.4 Static Semantics: CodePointAt ( string, position
)
The abstract operation CodePointAt takes arguments string (a String) and position (a
non-negative integer) and returns a Record with
fields [[CodePoint]] (a code point), [[CodeUnitCount]] (a
positive integer), and [[IsUnpairedSurrogate]] (a Boolean). It interprets string as a sequence of
UTF-16 encoded code points, as described in 6.1.4, and
reads from it a single code point starting with the code unit at index position. It performs the
following steps when called:
The abstract operation StringToCodePoints takes argument string (a String) and returns a
List of code
points. It returns the sequence of Unicode code points that results from interpreting string as
UTF-16 encoded Unicode text as described in 6.1.4. It
performs the following steps when called:
The abstract operation ParseText takes arguments sourceText (a sequence of Unicode code points)
and goalSymbol (a nonterminal in one of the ECMAScript grammars) and returns a Parse Node or a non-empty
List of
SyntaxError objects. It performs the following steps when called:
2. If the parse succeeded and no early errors were
found, return the Parse Node (an instance of
goalSymbol) at the root of the parse tree resulting from the parse.
3. Otherwise, return a List of
one or more SyntaxError objects representing the parsing errors and/or early
errors. If more than one parsing error or early error is
present, the number and ordering of error objects in the list is implementation-defined,
but at least one must be present.
Note 1
Consider a text that has an early error at a particular point,
and also a syntax error at a later point. An implementation that does a parse pass followed by an
early
errors pass might report the syntax error and not proceed to the early
errors pass. An implementation that interleaves the two activities might report the
early
error and not proceed to find the syntax error. A third implementation might report
both errors. All of these behaviours are conformant.
Eval code is the source text supplied to the built-in eval function.
More precisely, if the parameter to the built-in eval function is a
String, it is treated as an ECMAScript Script. The eval code for a particular
invocation of eval is the global code portion of that Script.
then the source text
matched by the BindingIdentifier (if any) of
that declaration or expression is also included in the function code of the corresponding function.
Function code is generally provided as the bodies of Function Definitions (15.2), Arrow Function
Definitions (15.3), Method
Definitions (15.4), Generator Function
Definitions (15.5), Async
Function Definitions (15.8), Async
Generator Function Definitions (15.6),
and Async Arrow Functions (15.9).
Function code is also derived from the arguments to the Function constructor (20.2.1.1), the
GeneratorFunction constructor (27.3.1.1), and the
AsyncFunction constructor (27.7.1.1).
Note 2
The practical effect of including the BindingIdentifier in function
code is that the Early Errors for strict mode code are applied
to a BindingIdentifier that is the
name of a function whose body contains a "use strict" directive, even if the surrounding code is not
strict mode code.
11.2.1 Directive Prologues and the Use Strict Directive
An ECMAScript syntactic unit may be processed using either unrestricted or strict mode syntax and semantics
(4.3.2). Code is
interpreted as strict mode code in the following situations:
ECMAScript code that is not strict mode code is called non-strict
code.
11.2.3 Non-ECMAScript Functions
An ECMAScript implementation may support the evaluation of function exotic objects whose
evaluative behaviour is expressed in some host-defined form of executable code
other than ECMAScript source text. Whether a
function object is defined within
ECMAScript code or is a built-in function is not observable from the perspective of ECMAScript code that
calls or is called by such a function object.
12 ECMAScript Language: Lexical Grammar
The source text of an ECMAScript Script or Module is first converted into a sequence of
input elements, which are tokens, line terminators, comments, or white space. The source text is scanned from
left to right, repeatedly taking the longest possible sequence of code points as the next input element.
The use of multiple lexical goals ensures that there are no lexical ambiguities that would affect automatic
semicolon insertion. For example, there are no syntactic grammar contexts where both a leading division or
division-assignment, and a leading RegularExpressionLiteral
are permitted. This is not affected by semicolon insertion (see 12.10); in
examples such as the following:
a = b
/hi/g.exec(c).map(d);
where the first non-whitespace, non-comment code point after a LineTerminator is U+002F (SOLIDUS)
and the syntactic context allows division or division-assignment, no semicolon is inserted at the LineTerminator.
That is, the above example is interpreted in the same way as:
The Unicode format-control characters (i.e., the characters in category “Cf” in the Unicode Character
Database such as LEFT-TO-RIGHT MARK or RIGHT-TO-LEFT MARK) are control codes used to control the formatting of
a range of text in the absence of higher-level protocols for this (such as mark-up languages).
It is useful to allow format-control characters in source text to facilitate editing and display. All format
control characters may be used within comments, and within string literals, template literals, and regular
expression literals.
U+200C (ZERO WIDTH NON-JOINER) and U+200D (ZERO WIDTH JOINER) are format-control characters that are used to
make necessary distinctions when forming words or phrases in certain languages. In ECMAScript
source text these code points may also be used in an IdentifierName after the first
character.
U+FEFF (ZERO WIDTH NO-BREAK SPACE) is a format-control character used primarily at the start of a text to
mark it as Unicode and to allow detection of the text's encoding and byte order. <ZWNBSP> characters
intended for this purpose can sometimes also appear after the start of a text, for example as a result of
concatenating files. In ECMAScript source text <ZWNBSP>
code points are treated as white space characters (see 12.2).
The special treatment of certain format-control characters outside of comments, string literals, and regular
expression literals is summarized in Table 35.
White space code points are used to improve source text readability and to separate tokens (indivisible
lexical units) from each other, but are otherwise insignificant. White space code points may occur between any
two tokens and at the start or end of input. White space code points may occur within a StringLiteral, a
RegularExpressionLiteral,
a Template, or a
TemplateSubstitutionTail
where they are considered significant code points forming part of a literal value. They may also occur within
a Comment, but
cannot appear within any other kind of token.
The ECMAScript white space code points are listed in Table 36.
Table 36: White Space Code Points
Code Points
Name
Abbreviation
U+0009
CHARACTER TABULATION
<TAB>
U+000B
LINE TABULATION
<VT>
U+000C
FORM FEED (FF)
<FF>
U+FEFF
ZERO WIDTH NO-BREAK SPACE
<ZWNBSP>
any code point in general category “Space_Separator”
<USP>
Note 1
U+0020 (SPACE) and U+00A0 (NO-BREAK SPACE) code points are part of <USP>.
Note 2
Other than for the code points listed in Table 36, ECMAScript
WhiteSpace
intentionally excludes all code points that have the Unicode “White_Space” property but which are not
classified in general category “Space_Separator” (“Zs”).
Like white space code points, line terminator code points are used to improve source text readability and to
separate tokens (indivisible lexical units) from each other. However, unlike white space code points, line
terminators have some influence over the behaviour of the syntactic grammar. In general, line terminators may
occur between any two tokens, but there are a few places where they are forbidden by the syntactic grammar.
Line terminators also affect the process of automatic semicolon insertion (12.10). A line
terminator cannot occur within any token except a StringLiteral, Template, or TemplateSubstitutionTail.
<LF> and <CR> line terminators cannot occur within a StringLiteral token except as part of
a LineContinuation.
Line terminators are included in the set of white space code points that are matched by the \s
class in regular expressions.
The ECMAScript line terminator code points are listed in Table
37.
Table 37: Line Terminator Code Points
Code Point
Unicode Name
Abbreviation
U+000A
LINE FEED (LF)
<LF>
U+000D
CARRIAGE RETURN (CR)
<CR>
U+2028
LINE SEPARATOR
<LS>
U+2029
PARAGRAPH SEPARATOR
<PS>
Only the Unicode code points in Table 37 are treated
as line terminators. Other new line or line breaking Unicode code points are not treated as line terminators
but are treated as white space if they meet the requirements listed in Table 36. The sequence
<CR><LF> is commonly used as a line terminator. It should be considered a single SourceCharacter
for the purpose of reporting line numbers.
Comments can be either single or multi-line. Multi-line comments cannot nest.
Because a single-line comment can contain any Unicode code point except a LineTerminator code point, and
because of the general rule that a token is always as long as possible, a single-line comment always consists
of all code points from the // marker to the end of the line. However, the LineTerminator at
the end of the line is not considered to be part of the single-line comment; it is recognized separately by
the lexical grammar and becomes part of the stream of input elements for the syntactic grammar. This point is
very important, because it implies that the presence or absence of single-line comments does not affect the
process of automatic semicolon insertion (see 12.10).
Comments behave like white space and are discarded except that, if a MultiLineComment contains a line
terminator code point, then the entire comment is considered to be a LineTerminator for purposes of
parsing by the syntactic grammar.
IdentifierName
and ReservedWord
are
tokens that are interpreted according to the Default Identifier Syntax given in Unicode Standard Annex #31,
Identifier and Pattern Syntax, with some small modifications. ReservedWord is an enumerated subset
of IdentifierName. The syntactic
grammar defines Identifier as an IdentifierName that
is not a ReservedWord. The Unicode identifier
grammar is based on character properties specified by the Unicode Standard. The Unicode code points in the
specified categories in the latest version of the Unicode Standard must be treated as in those categories by
all conforming ECMAScript implementations. ECMAScript implementations may recognize identifier code points
defined in later editions of the Unicode Standard.
Note 1
This standard specifies specific code point additions: U+0024 (DOLLAR SIGN) and U+005F (LOW LINE) are
permitted anywhere in an IdentifierName, and the code
points U+200C (ZERO WIDTH NON-JOINER) and U+200D (ZERO WIDTH JOINER) are permitted anywhere after the
first code point of an IdentifierName.
The sets of code points with Unicode properties “ID_Start” and “ID_Continue” include, respectively, the
code points with Unicode properties “Other_ID_Start” and “Other_ID_Continue”.
12.7.1 Identifier Names
Unicode escape sequences are permitted in an IdentifierName, where they
contribute a single Unicode code point equal to the IdentifierCodePoint of the
UnicodeEscapeSequence. The
\ preceding the UnicodeEscapeSequence does
not contribute any code points. A UnicodeEscapeSequence
cannot be used to contribute a code point to an IdentifierName that would
otherwise be invalid. In other words, if a \UnicodeEscapeSequence
sequence were replaced by the SourceCharacter it contributes,
the result must still be a valid IdentifierName that has the exact
same sequence of SourceCharacter elements as the
original IdentifierName. All
interpretations of IdentifierName within this
specification are based upon their actual code points regardless of whether or not an escape sequence was
used to contribute any particular code point.
Two IdentifierNames that are
canonically equivalent according to the Unicode Standard are not equal unless, after replacement of
each UnicodeEscapeSequence, they
are represented by the exact same sequence of code points.
The syntax-directed
operation IdentifierCodePoints takes no arguments and returns a List of
code points. It is defined piecewise over the following productions:
1. Return the code point whose numeric value is the MV of CodePoint.
12.7.2 Keywords and Reserved Words
A keyword is a token that matches IdentifierName, but also has a
syntactic use; that is, it appears literally, in a fixed width font, in some syntactic
production. The keywords of ECMAScript include if, while, async,
await, and many others.
A reserved word is an IdentifierName that cannot be used
as an identifier. Many keywords are reserved words, but some are not, and some are reserved only in certain
contexts. if and while are reserved words. await is reserved only
inside async functions and modules. async is not reserved; it can be used as a variable name or
statement label without restriction.
This specification uses a combination of grammatical productions and early error rules to
specify which names are valid identifiers and which are reserved words. All tokens in the ReservedWord list
below, except for await and yield, are unconditionally reserved. Exceptions for
await and yield are specified in 13.1, using
parameterized syntactic productions. Lastly, several early error rules restrict the set of
valid identifiers. See 13.1.1,
14.3.1.1,
14.7.5.1,
and 15.7.1.
In summary, there are five categories of identifier names:
Those that are always allowed as identifiers, and are not keywords, such as Math,
window, toString, and _;
Those that are never allowed as identifiers, namely the ReservedWords listed below
except await and yield;
Those that are contextually allowed as identifiers, namely await and yield;
Those that are contextually disallowed as identifiers, in strict mode
code: let, static, implements,
interface, package, private, protected, and
public;
Those that are always allowed as identifiers, but also appear as keywords within certain syntactic
productions, at places where Identifier is not allowed:
as, async, from, get, meta,
of, set, and target.
The term conditional keyword, or contextual keyword, is sometimes used to refer to the
keywords that fall in the last three categories, and thus can be used as identifiers in some contexts and as
keywords in others.
Per 5.1.5, keywords in the
grammar match literal sequences of specific SourceCharacter elements. A
code point in a keyword cannot be expressed by a \UnicodeEscapeSequence.
enum is not currently used as a keyword in this specification. It is a future reserved
word, set aside for use as a keyword in future language extensions.
Similarly, implements, interface, package, private,
protected, and public are future reserved words in strict mode code.
The syntax-directed
operation NumericValue takes no arguments and returns a Number or a BigInt. It is
defined piecewise over the following productions:
A string literal is 0 or more Unicode code points enclosed in single or double quotes. Unicode code
points may also be represented by an escape sequence. All code points may appear literally in a string
literal except for the closing quote code points, U+005C (REVERSE SOLIDUS), U+000D (CARRIAGE RETURN),
and U+000A (LINE FEED). Any code points may appear in the form of an escape sequence. String literals
evaluate to ECMAScript String values. When generating these String values Unicode code points are UTF-16
encoded as defined in 11.1.1. Code points
belonging to the Basic Multilingual Plane are encoded as a single code unit element of the string. All
other code points are encoded as two code unit elements of the string.
<LF> and <CR> cannot appear in a string literal, except as part of a LineContinuation to produce
the empty code points sequence. The proper way to include either in the String value of a string literal
is to use an escape sequence such as \n or \u000A.
It is possible for string literals to precede a Use Strict
Directive that places the enclosing code in strict
mode, and implementations must take care to enforce the above rules for such
literals. For example, the following source text contains a Syntax Error:
A string literal stands for a value of the String
type. SV produces String values for string literals through recursive application on
the various parts of the string literal. As part of this process, some Unicode code points within the
string literal are interpreted as having a mathematical value, as
described below or in 12.9.3.
A regular expression literal is an input element that is converted to a RegExp object (see 22.2) each
time the literal is evaluated. Two regular expression literals in a program evaluate to regular
expression objects that never compare as === to each other even if the two literals'
contents are identical. A RegExp object may also be created at runtime by new RegExp or
calling the RegExp constructor as a function (see
22.2.4).
The productions below describe the syntax for a regular expression literal and are used by the input
element scanner to find the end of the regular expression literal. The source text comprising the RegularExpressionBody and
the RegularExpressionFlags are
subsequently parsed again using the more stringent ECMAScript Regular Expression grammar (22.2.1).
An implementation may extend the ECMAScript Regular Expression grammar defined in 22.2.1,
but it must not extend the
RegularExpressionBody and
RegularExpressionFlags
productions defined below or the productions used by these productions.
Regular expression literals may not be empty; instead of representing an empty regular expression
literal, the code unit sequence // starts a single-line comment. To specify an empty
regular expression, use: /(?:)/.
12.9.5.1 Static Semantics: BodyText
The syntax-directed
operation BodyText takes no arguments and returns source text. It is defined piecewise
over the following productions:
The syntax-directed
operation TV takes no arguments and returns a String or undefined. A
template literal component is interpreted by TV as a value of the String
type. TV is used to construct the indexed components of a template object
(colloquially, the template values). In TV, escape sequences are replaced by the UTF-16 code unit(s) of
the Unicode code point represented by the escape sequence.
The syntax-directed
operation TRV takes no arguments and returns a String. A template literal component is
interpreted by TRV as a value of the String
type. TRV is used to construct the raw components of a template object (colloquially,
the template raw values). TRV is similar to TV with the difference
being that in TRV, escape sequences are interpreted as they appear in the literal.
The TRV of HexDigit::one of0123456789abcdefABCDEF is the result of performing UTF16EncodeCodePoint on
the single code point matched by this production.
Most ECMAScript statements and declarations must be terminated with a semicolon. Such semicolons may always
appear explicitly in the source text. For convenience, however, such semicolons may be omitted from the source
text in certain situations. These situations are described by saying that semicolons are automatically
inserted into the source code token stream in those situations.
12.10.1 Rules of Automatic Semicolon Insertion
In the following rules, “token” means the actual recognized lexical token determined using the current
lexical goal symbol as described
in clause 12.
There are three basic rules of semicolon insertion:
When, as the source text is parsed from left to right, a token (called the offending token) is
encountered that is not allowed by any production of the grammar, then a semicolon is automatically
inserted before the offending token if one or more of the following conditions is true:
The offending token is separated from the previous token by at least one LineTerminator.
The offending token is }.
The previous token is ) and the inserted semicolon would then be parsed as the
terminating semicolon of a do-while statement (14.7.2).
When, as the source text is parsed from left to right, the end of the input stream of tokens is
encountered and the parser is unable to parse the input token stream as a single instance of the goal
nonterminal, then a semicolon is automatically inserted at the end of the input stream.
When, as the source text is parsed from left to right, a token is encountered that is allowed by some
production of the grammar, but the production is a restricted production and the token would be
the first token for a terminal or nonterminal immediately following the annotation “[no LineTerminator
here]” within the restricted production (and therefore such a token is called a restricted token), and the
restricted token is separated from the previous token by at least one LineTerminator, then a semicolon
is automatically inserted before the restricted token.
However, there is an additional overriding condition on the preceding rules: a semicolon is never inserted
automatically if the semicolon would then be parsed as an empty statement or if that semicolon would become
one of the two semicolons in the header of a for statement (see 14.7.4).
Note
The following are the only restricted productions in the grammar:
The practical effect of these restricted productions is as follows:
When a ++ or -- token is encountered where the parser would treat it as a
postfix operator, and at least one LineTerminator occurred
between the preceding token and the ++ or -- token, then a semicolon is
automatically inserted before the ++ or -- token.
When a continue, break, return, throw, or
yield token is encountered and a LineTerminator is
encountered before the next token, a semicolon is automatically inserted after the
continue, break, return, throw, or
yield token.
When arrow function parameter(s) are followed by a LineTerminator before a
=> token, a semicolon is automatically inserted and the punctuator causes a syntax
error.
When an async token is followed by a LineTerminator before a
function or IdentifierName or
( token, a semicolon is automatically inserted and the async token is not
treated as part of the same expression or class element as the following tokens.
When an async token is followed by a LineTerminator before a
* token, a semicolon is automatically inserted and the punctuator causes a syntax error.
The resulting practical advice to ECMAScript programmers is:
A postfix ++ or -- operator should be on the same line as its operand.
An Expression
in a
return or throw statement or an AssignmentExpression
in a yield expression should start on the same line as the return,
throw, or yield token.
A LabelIdentifier in a
break or continue statement should be on the same line as the
break or continue token.
The end of an arrow function's parameter(s) and its => should be on the same line.
The async token preceding an asynchronous function or method should be on the same line
as the immediately following token.
12.10.2 Examples of Automatic Semicolon Insertion
This section is non-normative.
The source
{ 12 } 3
is not a valid sentence in the ECMAScript grammar, even with the automatic semicolon insertion rules. In
contrast, the source
{ 12 } 3
is also not a valid ECMAScript sentence, but is transformed by automatic semicolon insertion into the
following:
{ 1
;2 ;} 3;
which is a valid ECMAScript sentence.
The source
for (a; b
)
is not a valid ECMAScript sentence and is not altered by automatic semicolon insertion because the
semicolon is needed for the header of a for statement. Automatic semicolon insertion never
inserts one of the two semicolons in the header of a for statement.
The source
return
a + b
is transformed by automatic semicolon insertion into the following:
return;
a + b;
Note 1
The expression a + b is not treated as a value to be returned by the return
statement, because a LineTerminator separates it
from the token return.
The source
a = b
++c
is transformed by automatic semicolon insertion into the following:
a = b;
++c;
Note 2
The token ++ is not treated as a postfix operator applying to the variable b,
because a LineTerminator occurs between
b and ++.
The source
if (a > b)
else c = d
is not a valid ECMAScript sentence and is not altered by automatic semicolon insertion before the
else token, even though no production of the grammar applies at that point, because an
automatically inserted semicolon would then be parsed as an empty statement.
The source
a = b + c
(d + e).print()
is not transformed by automatic semicolon insertion, because the parenthesized expression that
begins the second line can be interpreted as an argument list for a function call:
a = b + c(d + e).print()
In the circumstance that an assignment statement must begin with a left parenthesis, it is a good idea for
the programmer to provide an explicit semicolon at the end of the preceding statement rather than to rely on
automatic semicolon insertion.
12.10.3 Interesting Cases of Automatic Semicolon Insertion
This section is non-normative.
ECMAScript programs can be written in a style with very few semicolons by relying on automatic semicolon
insertion. As described above, semicolons are not inserted at every newline, and automatic semicolon
insertion can depend on multiple tokens across line terminators.
As new syntactic features are added to ECMAScript, additional grammar productions could be added that cause
lines relying on automatic semicolon insertion preceding them to change grammar productions when parsed.
For the purposes of this section, a case of automatic semicolon insertion is considered interesting if it
is a place where a semicolon may or may not be inserted, depending on the source text which precedes it. The
rest of this section describes a number of interesting cases of automatic semicolon insertion in this
version of ECMAScript.
12.10.3.1 Interesting Cases of Automatic Semicolon Insertion in Statement
Lists
In a StatementList, many StatementListItems end in
semicolons, which may be omitted using automatic semicolon insertion. As a consequence of the rules above,
at the end of a line ending an expression, a semicolon is required if the following line begins with any
of the following:
An opening parenthesis ((). Without a semicolon, the two lines together
are treated as a CallExpression.
An opening square bracket ([). Without a semicolon, the two lines
together are treated as property access, rather than an ArrayLiteral or ArrayAssignmentPattern.
A template literal (`). Without a semicolon, the two lines together are
interpreted as a tagged Template (13.3.11), with the previous
expression as the MemberExpression.
Unary + or -. Without a semicolon, the two lines together
are interpreted as a usage of the corresponding binary operator.
A RegExp literal. Without a semicolon, the two lines together may be parsed instead
as the /MultiplicativeOperator,
for example if the RegExp has flags.
12.10.3.2 Cases of Automatic Semicolon Insertion and “[no LineTerminator
here]”
This section is non-normative.
ECMAScript contains grammar productions which include “[no LineTerminator here]”. These
productions are sometimes a means to have optional operands in the grammar. Introducing a LineTerminator
in these locations would change the grammar production of a source text by using the grammar production
without the optional operand.
The rest of this section describes a number of productions using “[no LineTerminator here]” in this
version of ECMAScript.
12.10.3.2.1 List of Grammar Productions with Optional Operands and “[no
LineTerminator here]”
yield and await are permitted as BindingIdentifier in the
grammar, and prohibited with static semantics below,
to prohibit automatic semicolon insertion in cases such as
It is a Syntax Error if this phrase is contained in strict mode
code and the StringValue of
IdentifierName is one of
"implements", "interface", "let",
"package", "private", "protected",
"public", "static", or "yield".
An ArrayLiteral is an expression
describing the initialization of an Array, using a list, of zero or more expressions each of which
represents an array element, enclosed in square brackets. The elements need not be literals; they are
evaluated each time the array initializer is evaluated.
Array elements may be elided at the beginning, middle or end of the element list. Whenever a comma in the
element list is not preceded by an AssignmentExpression (i.e.,
a comma at the beginning or after another comma), the missing array element contributes to the length of the
Array and increases the index of subsequent elements. Elided array elements are not defined. If an element
is elided at the end of an array, that element does not contribute to the length of the Array.
CreateDataPropertyOrThrow
is used to ensure that own properties are defined for the array even if the standard built-in
Array prototype
object has been modified in a manner that would preclude the creation of new own
properties using [[Set]].
An object initializer is an expression describing the initialization of an Object, written in a form
resembling a literal. It is a list of zero or more pairs of property keys
and associated values, enclosed in curly brackets. The values need not be literals; they are evaluated
each time the object initializer is evaluated.
In certain contexts, ObjectLiteral is used as a
cover grammar for a more restricted secondary grammar. The CoverInitializedName
production is necessary to fully cover these secondary grammars. However, use of this production results
in an early Syntax Error in normal contexts where an actual ObjectLiteral is expected.
It is a Syntax Error if any source text is matched by this production.
Note 1
This production exists so that ObjectLiteral can serve as a
cover grammar for ObjectAssignmentPattern.
It cannot occur in an actual object initializer.
The syntax-directed
operation PropertyNameList takes no arguments and returns a List of
Strings. It is defined piecewise over the following productions:
The abstract operation IsValidRegularExpressionLiteral takes argument literal (a RegularExpressionLiteralParse Node) and returns a
Boolean. It determines if its argument is a valid regular expression literal. It performs the following
steps when called:
b. Set patternText to the sequence of code points
resulting from interpreting each of the 16-bit elements of stringValue as a Unicode BMP
code point. UTF-16 decoding is not applied to the elements.
7. Let parseResult be ParsePattern(patternText,
u, v).
8. If parseResult is a Parse Node, return
true; else return false.
It is a Syntax Error if the number of elements in the result of TemplateStrings
of TemplateLiteral with argument
false is greater than or equal to 2**32.
The syntax-directed
operation TemplateStrings takes argument raw (a Boolean) and returns a
List of
either Strings or undefined. It is defined piecewise over the following productions:
This operation returns undefined if raw is false and
templateToken contains a NotEscapeSequence. In all
other cases, it returns a String.
13.2.8.4 GetTemplateObject ( templateLiteral )
The abstract operation GetTemplateObject takes argument templateLiteral (a Parse Node) and returns an
Array. It performs the following steps when called:
16. Append the Record
{ [[Site]]: templateLiteral, [[Array]]:
template } to realm.[[TemplateMap]].
17. Return template.
Note 1
The creation of a template object cannot result in an abrupt
completion.
Note 2
Each TemplateLiteral in the
program code of a realm is associated with a unique
template object that is used in the evaluation of tagged Templates (13.2.8.6).
The template objects are frozen and the same template object is used each time a specific tagged
Template is evaluated. Whether template objects are created lazily upon first evaluation of the
TemplateLiteral or eagerly
prior to first evaluation is an implementation choice that is not observable to ECMAScript code.
Note 3
Future editions of this specification may define additional non-enumerable properties of template
objects.
This algorithm does not apply GetValue to Evaluation of Expression. The
principal motivation for this is so that operators such as delete and typeof
may be applied to parenthesized expressions.
The abstract operation EvaluatePropertyAccessWithIdentifierKey takes arguments baseValue (an
ECMAScript language
value), identifierName (an IdentifierNameParse Node), and
strict (a Boolean) and returns a Reference
Record. It performs the following steps when called:
1. Let propertyNameString be StringValue of
identifierName.
2. Return the Reference
Record { [[Base]]: baseValue, [[ReferencedName]]: propertyNameString, [[Strict]]: strict, [[ThisValue]]:
empty }.
The abstract operation ContinueDynamicImport takes arguments promiseCapability (a PromiseCapability
Record) and moduleCompletion (either a normal completion
containing a Module Record or a
throw
completion) and returns unused. It completes the process of a
dynamic import originally started by an import() call,
resolving or rejecting the promise returned by that call as appropriate. It performs the following steps
when called:
a. Perform ! Call(promiseCapability.[[Reject]], undefined, «
moduleCompletion.[[Value]] »).
b. Return unused.
2. Let module be moduleCompletion.[[Value]].
3. Let loadPromise be
module.LoadRequestedModules().
4. Let rejectedClosure be a new Abstract Closure with
parameters (reason) that captures promiseCapability and performs the following
steps when called:
a. Perform ! Call(promiseCapability.[[Reject]], undefined, « reason »).
6. Let linkAndEvaluateClosure be a new Abstract Closure with
no parameters that captures module, promiseCapability, and
onRejected and performs the following steps when called:
i. Perform ! Call(promiseCapability.[[Reject]], undefined, « link.[[Value]] »).
ii. Return unused.
c. Let evaluatePromise be
module.Evaluate().
d. Let fulfilledClosure be a new Abstract Closure
with no parameters that captures module and promiseCapability and performs
the following steps when called:
A tagged template is a function call where the arguments of the call are derived from a TemplateLiteral
(13.2.8). The actual
arguments include a template object (13.2.8.4) and the values
produced by evaluating the expressions embedded within the TemplateLiteral.
The host-defined abstract operation
HostFinalizeImportMeta takes arguments importMeta (an Object) and moduleRecord (a
Module Record) and
returns unused. It allows hosts to perform any extraordinary
operations to prepare the object returned from import.meta.
Most hosts will be able to simply define
HostGetImportMetaProperties,
and leave HostFinalizeImportMeta with its default behaviour. However, HostFinalizeImportMeta provides an
"escape hatch" for hosts which need to directly manipulate the
object before it is exposed to ECMAScript code.
The default implementation of HostFinalizeImportMeta is to return unused.
c. Return ? base.DeleteBinding(ref.[[ReferencedName]]).
Note 1
When a delete operator occurs within strict mode
code, a SyntaxError exception is thrown if its UnaryExpression is a direct
reference to a variable, function argument, or function name. In addition, if a delete
operator occurs within strict mode code and the
property to be deleted has the attribute { [[Configurable]]:
false } (or otherwise cannot be deleted), a TypeError exception
is thrown.
Note 2
The object that may be created in step 4.c
is not
accessible outside of the above abstract operation and the ordinary
object[[Delete]] internal method. An implementation might
choose to avoid the actual creation of that object.
The result of evaluating a relational operator is always of type Boolean, reflecting whether the
relationship named by the operator holds between its two operands.
The abstract operation InstanceofOperator takes arguments V (an ECMAScript language
value) and target (an ECMAScript language
value) and returns either a normal
completion containing a Boolean or a throw
completion. It implements the generic algorithm for determining if V is an
instance of target either by consulting target's @@hasInstance method or, if
absent, determining whether the value of target's "prototype" property is
present in V's prototype chain. It performs the following steps when called:
Steps 4 and 5 provide compatibility
with previous editions of ECMAScript that did not use a @@hasInstance
method to
define the instanceof operator semantics. If an object does not define or inherit @@hasInstance it uses the
default instanceof semantics.
13.11 Equality Operators
Note
The result of evaluating an equality operator is always of type Boolean, reflecting whether the
relationship named by the operator holds between its two operands.
6. If r is true, return
false. Otherwise, return true.
Note 1
Given the above definition of equality:
String comparison can be forced by: `${a}` == `${b}`.
Numeric comparison can be forced by: +a == +b.
Boolean comparison can be forced by: !a == !b.
Note 2
The equality operators maintain the following invariants:
A != B is equivalent to !(A == B).
A == B is equivalent to B == A, except in the order of evaluation of
A and B.
Note 3
The equality operator is not always transitive. For example, there might be two distinct String
objects, each representing the same String value; each String object would be considered equal to the
String value by the == operator, but the two String objects would not be equal to each
other. For example:
new String("a") == "a" and "a" == new String("a") are both
true.
new String("a") == new String("a") is false.
Note 4
Comparison of Strings uses a simple equality test on sequences of code unit values. There is no attempt
to use the more complex, semantically oriented definitions of character or string equality and collating
order defined in the Unicode specification. Therefore Strings values that are canonically equal
according to the Unicode Standard could test as unequal. In effect this algorithm assumes that both
Strings are already in normalized form.
The value produced by a && or || operator is not necessarily of type
Boolean. The value produced will always be the value of one of the two operand expressions.
The grammar for a ConditionalExpression in
ECMAScript is slightly different from that in C and Java, which each allow the second subexpression to be
an Expression
but restrict the third expression to be a ConditionalExpression.
The motivation for this difference in ECMAScript is to allow an assignment expression to be governed by
either arm of a conditional and to eliminate the confusing and fairly useless case of a comma expression
as the centre expression.
When this expression occurs within strict mode code, it is a
runtime error if lref in step 1.d,
2,
2,
2,
2
is an unresolvable reference. If it is, a ReferenceError exception is thrown.
Additionally, it is a runtime error if the lref in step 8,
7,
7,
6
is a reference to a data property with the attribute
value { [[Writable]]: false }, to an accessor
property with the
attribute value { [[Set]]: undefined }, or to a non-existent
property of an object for which the IsExtensible predicate
returns the value false. In these cases a TypeError exception is
thrown.
No hint is provided in the calls to ToPrimitive in steps 1.a and
1.b. All
standard objects except Dates handle the absence of a hint as if number were
given; Dates handle the absence of a hint as if string were given. Exotic
objects may handle the absence of a hint in some other manner.
Note 2
Step 1.c differs from
step 3 of the IsLessThan algorithm, by using
the logical-or operation instead of the logical-and operation.
The abstract operation EvaluateStringOrNumericBinaryExpression takes arguments leftOperand (a
Parse Node), opText
(a sequence of Unicode code points), and rightOperand (a Parse Node)
and returns either a normal completion
containing either a String, a BigInt, or a Number, or an abrupt
completion. It performs the following steps when called:
Left to right evaluation order is maintained by evaluating a DestructuringAssignmentTarget
that is not a destructuring pattern prior to accessing the iterator or evaluating the Initializer.
The value of a StatementList is the value of
the last value-producing item in the StatementList. For example, the
following calls to the eval function all return the value 1:
The abstract operation BlockDeclarationInstantiation takes arguments code (a Parse Node) and env
(a Declarative Environment
Record) and returns unused. code is the Parse Node corresponding to
the body of the block. env is the Environment Record in which
bindings are to be created.
When undefined is passed for environment it indicates that a PutValue operation should be used
to assign the initialization value. This is the case for formal parameter lists of non-strict functions. In
that case the formal parameter bindings are preinitialized in order to deal with the possibility of
multiple parameters with the same name.
It is defined piecewise over the following productions:
The lookahead-restriction [lookahead ≠ else] resolves the classic
"dangling else" problem in the usual way. That is, when the choice of associated if is
otherwise ambiguous, the else is associated with the nearest (innermost) of the candidate
ifs
The abstract operation LoopContinues takes arguments completion (a Completion
Record) and labelSet (a List of
Strings) and returns a Boolean. It performs the following steps when called:
The abstract operation CreatePerIterationEnvironment takes argument perIterationBindings (a
List of
Strings) and returns either a normal completion
containingunused or a throw
completion. It performs the following steps when called:
undefined is passed for environment to indicate that a PutValue operation should be used
to assign the initialization value. This is the case for var statements and the formal
parameter lists of some non-strict functions (see
10.2.11).
In those cases a lexical binding is hoisted and preinitialized prior to evaluation of its initializer.
It is defined piecewise over the following productions:
The abstract operation ForIn/OfBodyEvaluation takes arguments lhs (a Parse Node), stmt
(a StatementParse Node),
iteratorRecord (an Iterator Record),
iterationKind (enumerate or iterate),
lhsKind (assignment, var-binding, or
lexical-binding), and labelSet (a List of
Strings) and optional argument iteratorKind (sync or
async) and returns either a normal completion
containing an ECMAScript language
value or an abrupt
completion. It performs the following steps when called:
1. If iteratorKind is not present, set
iteratorKind to sync.
The abstract operation EnumerateObjectProperties takes argument O (an Object) and returns an
Iterator. It performs the following steps when called:
1. Return an Iterator object (27.1.1.2) whose
next method iterates over all the String-valued keys of enumerable properties of
O. The iterator object is never directly accessible to ECMAScript code. The mechanics and
order of enumerating the properties is not specified but must conform to the rules specified below.
The iterator's throw and return methods are null and are
never invoked. The iterator's next method processes object properties to determine whether
the property key should be returned as
an iterator value. Returned property keys do not include keys
that are Symbols. Properties of the target object may be deleted during enumeration. A property that is
deleted before it is processed by the iterator's next method is ignored. If new properties
are added to the target object during enumeration, the newly added properties are not guaranteed to be
processed in the active enumeration. A property name will be returned by
the iterator's next method at most once in any enumeration.
Enumerating the properties of the target object includes enumerating properties of its prototype, and the
prototype of the prototype, and so on, recursively; but a property of a prototype is not processed if it
has the same name as a property that has already been processed by the iterator's next
method. The values of [[Enumerable]] attributes are not considered when
determining if a property of a prototype object has already been processed. The enumerable property names
of prototype objects must be obtained by invoking EnumerateObjectProperties passing the prototype object
as the argument. EnumerateObjectProperties must obtain the own property keys of
the target object by calling its [[OwnPropertyKeys]] internal method. Property
attributes of the target object must be obtained by calling its [[GetOwnProperty]] internal method.
the value of the [[Prototype]] internal slot of O or an object in
its prototype chain changes,
a property is removed from O or an object in its prototype chain,
a property is added to an object in O's prototype chain, or
the value of the [[Enumerable]] attribute of a property of O or an
object in its prototype chain changes.
Note 1
ECMAScript implementations are not required to implement the algorithm in 14.7.5.10.2.1
directly. They may choose any implementation whose behaviour will not deviate from that algorithm
unless one of the constraints in the previous paragraph is violated.
The following is an informative definition of an ECMAScript generator function that conforms to these
rules:
function* EnumerateObjectProperties(obj) {
const visited = newSet();
for (const key ofReflect.ownKeys(obj)) {
if (typeof key === "symbol") continue;
const desc = Reflect.getOwnPropertyDescriptor(obj, key);
if (desc) {
visited.add(key);
if (desc.enumerable) yield key;
}
}
const proto = Reflect.getPrototypeOf(obj);
if (proto === null) return;
for (const protoKey ofEnumerateObjectProperties(proto)) {
if (!visited.has(protoKey)) yield protoKey;
}
}
Note 2
The list of exotic objects for which
implementations are not required to match CreateForInIterator was
chosen because implementations historically differed in behaviour for those cases, and agreed in all
others.
14.7.5.10 For-In Iterator Objects
A For-In Iterator is an object that represents a specific iteration over some specific object. For-In
Iterator objects are never directly accessible to ECMAScript code; they exist solely to illustrate the
behaviour of EnumerateObjectProperties.
14.7.5.10.1 CreateForInIterator ( object )
The abstract operation CreateForInIterator takes argument object (an Object) and returns a
For-In Iterator. It is used to create a For-In Iterator object which iterates over the own and inherited
enumerable string properties of object in a specific order. It performs the following steps
when called:
14.7.5.10.3 Properties of For-In Iterator Instances
For-In Iterator instances are ordinary objects that inherit
properties from the %ForInIteratorPrototype%
intrinsic object. For-In Iterator instances are initially created with the internal slots listed in
Table 39.
Table 39: Internal Slots of For-In Iterator Instances
Internal Slot
Type
Description
[[Object]]
an Object
The Object value whose properties are being iterated.
[[ObjectWasVisited]]
a Boolean
true if the iterator has invoked [[OwnPropertyKeys]] on [[Object]],
false otherwise.
It is a Syntax Error if this ContinueStatement is not
nested, directly or indirectly (but not crossing function or static initialization block
boundaries), within an IterationStatement.
It is a Syntax Error if this BreakStatement is not nested,
directly or indirectly (but not crossing function or static initialization block boundaries),
within an IterationStatement or a
SwitchStatement.
A return statement causes a function to cease execution and, in most cases, returns a value
to the caller. If Expression is omitted, the return
value is undefined. Otherwise, the return value is the value of Expression. A
return statement may not actually return a value to the caller depending on surrounding
context. For example, in a try block, a return statement's Completion
Record may be replaced with another Completion
Record during evaluation of the finally block.
The with statement adds an Object Environment
Record for a computed object to the lexical environment of the running execution
context. It then executes a statement using this augmented lexical environment.
Finally, it restores the original lexical environment.
No matter how control leaves the embedded Statement, whether normally or by
some form of abrupt
completion or exception, the LexicalEnvironment is always restored to its former
state.
This operation does not execute C's StatementList (if any). The
CaseBlock
algorithm uses its return value to determine which StatementList to start
executing.
A Statement
may be prefixed by a label. Labelled statements are only used in conjunction with labelled
break and continue statements. ECMAScript has no goto statement. A
Statement can
be part of a LabelledStatement, which
itself can be part of a LabelledStatement, and so on.
The labels introduced this way are collectively referred to as the “current label set” when describing the
semantics of individual statements.
The abstract operation IsLabelledFunction takes argument stmt (a StatementParse Node) and returns a
Boolean. It performs the following steps when called:
The try statement encloses a block of code in which an exceptional condition can occur, such
as a runtime error or a throw statement. The catch clause provides the
exception-handling code. When a catch clause catches an exception, its CatchParameter is bound to that
exception.
Evaluating a DebuggerStatement may allow
an implementation to cause a breakpoint when run under a debugger. If a debugger is not present or
active this statement has no observable effect.
Various ECMAScript language elements cause the creation of ECMAScript function objects
(10.2). Evaluation of such functions starts
with the execution of their [[Call]] internal method (10.2.1).
The ExpectedArgumentCount of a FormalParameterList is
the number of FormalParameters to the left
of either the rest parameter or the first FormalParameter with an
Initializer. A FormalParameter without an
initializer is allowed after the first parameter with an initializer but such parameters are considered
to be optional with undefined as their default value.
The syntax-directed
operation FunctionBodyContainsUseStrict takes no arguments and returns a Boolean. It is
defined piecewise over the following productions:
A "prototype" property is automatically created for every function defined using a
FunctionDeclaration or
FunctionExpression, to
allow for the possibility that the function will be used as a constructor.
The syntax-directed
operation ConciseBodyContainsUseStrict takes no arguments and returns a Boolean. It is
defined piecewise over the following productions:
An ArrowFunction does not define
local bindings for arguments, super, this, or
new.target. Any reference to arguments, super, this,
or new.target within an ArrowFunction must resolve to a
binding in a lexically enclosing environment. Typically this will be the Function Environment of an
immediately enclosing function. Even though an ArrowFunction may contain
references to super, the function object created in step
5
is not made into a method by performing MakeMethod. An ArrowFunction
that references super is always contained within a non-ArrowFunction and the necessary
state to implement super is accessible via the env that is captured by the
function object of the ArrowFunction.
YieldExpression cannot be used
within the FormalParameters of a
generator function because any expressions that are part of FormalParameters are evaluated
before the resulting Generator is in a resumable state.
2. Let G be ? OrdinaryCreateFromConstructor(functionObject,
"%GeneratorFunction.prototype.prototype%", « [[GeneratorState]], [[GeneratorContext]], [[GeneratorBrand]] »).
1. Let innerResult be ? Call(throw, iterator, «
received.[[Value]] »).
2. If generatorKind is
async, set innerResult to ? Await(innerResult).
3. NOTE: Exceptions from the inner iterator
throw method are propagated. Normal
completions from an inner throw method are processed
similarly to an inner next.
1. NOTE: If iterator does not have a
throw method, this throw is going to terminate the yield* loop.
But first we need to give iterator a chance to clean up.
2. Let closeCompletion be Completion
Record { [[Type]]:
normal, [[Value]]:
empty, [[Target]]:
empty }.
3. If generatorKind is
async, perform ? AsyncIteratorClose(iteratorRecord,
closeCompletion).
It is a Syntax Error if PrivateBoundIdentifiers
of ClassElementList contains any
duplicate entries, unless the name is used once for a getter and once for a setter and in no other
entries, and the getter and setter are either both static or both non-static.
The syntax-directed
operation ClassElementKind takes no arguments and returns
constructor-method, non-constructor-method, or
empty. It is defined piecewise over the following productions:
The syntax-directed
operation AllPrivateIdentifiersValid takes argument names (a List of
Strings) and returns a Boolean.
Every grammar production alternative in this specification which is not listed below implicitly has the
following default definition of AllPrivateIdentifiersValid:
1. For each child node child of this Parse Node, do
The syntax-directed
operation PrivateBoundIdentifiers takes no arguments and returns a List of
Strings. It is defined piecewise over the following productions:
Every grammar production alternative in this specification which is not listed below implicitly has the
following default definition of ContainsArguments:
1. For each child node child of this Parse Node, do
For ease of specification, private methods and accessors are included alongside private fields in the
[[PrivateElements]] slot of class instances. However, any given object has
either all or none of the private methods and accessors defined by a given class. This feature has been
designed so that implementations may choose to implement private methods and accessors using a strategy
which does not require tracking each method or accessor individually.
For example, an implementation could directly associate instance private methods with their
corresponding Private Name and track, for
each object, which class constructors have run with that
object as their this value. Looking up an instance private method on an object then
consists of checking that the class constructor which defines the method
has been used to initialize the object, then returning the method associated with the Private
Name.
This differs from private fields: because field initializers can throw during class instantiation, an
individual object may have some proper subset of the private fields of a given class, and so private
fields must in general be tracked individually.
It is defined piecewise over the following productions:
1. NOTE: This branch behaves similarly to
constructor(...args) { super(...args); }. The most notable distinction is
that while the aforementioned ECMAScript source
text observably calls the @@iterator
method on %Array.prototype%, this function does not.
2. Let func be ! F.[[GetPrototypeOf]]().
3. If IsConstructor(func)
is false, throw a TypeError exception.
4. Let result be ? Construct(func, args,
NewTarget).
v. Else,
1. NOTE: This branch behaves similarly to
constructor() {}.
await is parsed as a keyword of an
AwaitExpression when the
[Await] parameter is present. The [Await] parameter is present in the top level of
the following contexts, although the parameter may be absent in some contexts depending on the
nonterminals, such as FunctionBody:
When Script is
the syntactic goal symbol,
await may be parsed as an identifier when the [Await] parameter is absent. This
includes the following contexts:
The syntax-directed
operation AsyncConciseBodyContainsUseStrict takes no arguments and returns a Boolean. It
is defined piecewise over the following productions:
Tail Position calls are only defined in strict mode code because of
a common non-standard language extension (see 10.2.4) that
enables observation of the chain of caller contexts.
call is a Parse Node that represents
a specific range of source text. When the following algorithms compare call to another
Parse Node, it is a test
of whether they represent the same source text.
Note 2
A potential tail position call that is immediately followed by return GetValue of the
call result is also a possible tail position call. A function call cannot return a Reference
Record, so such a GetValue operation will always
return the same value as the actual function call result.
It is defined piecewise over the following productions:
The abstract operation PrepareForTailCall takes no arguments and returns unused. It
performs the following steps when called:
1. Assert: The current execution context will
not subsequently be used for the evaluation of any ECMAScript code or built-in functions. The invocation
of Call subsequent to the invocation of this abstract operation will create and push a new execution context before
performing any such evaluation.
2. Discard all resources associated with the current execution context.
3. Return unused.
A tail position call must either release any transient internal resources associated with the currently
executing function execution context before
invoking the target function or reuse those resources in support of the target function.
Note
For example, a tail position call should only grow an implementation's activation record stack by the
amount that the size of the target function's activation record exceeds the size of the calling
function's activation record. If the target function's activation record is smaller, then the total size
of the stack should decrease.
A map from the specifier strings imported by this script to the resolved Module Record.
The list does not contain two different Records
with the same [[Specifier]].
[[HostDefined]]
anything (default value is empty)
Field reserved for use by host environments that
need to associate additional information with a script.
The abstract operation ParseScript takes arguments sourceText (ECMAScript source
text), realm (a Realm Record or
undefined), and hostDefined (anything) and returns a Script
Record or a non-empty List of
SyntaxError objects. It creates a Script Record based upon the result of
parsing sourceText as a Script. It performs the following steps
when called:
An implementation may parse script source text and analyse it for Early Error conditions prior to
evaluation of ParseScript for that script source text. However, the reporting of any errors must be
deferred until the point where this specification actually performs ParseScript upon that source text.
When an execution context is
established for evaluating scripts, declarations are instantiated in the current global environment.
Each global binding declared in the code is instantiated.
1. If declaredFunctionNames does not contain
vn, then
a. Let vnDefinable be
? env.CanDeclareGlobalVar(vn).
b. If vnDefinable is
false, throw a TypeError exception.
c. If declaredVarNames does not contain
vn, then
i. Append vn to
declaredVarNames.
11. NOTE: No abnormal terminations occur after this algorithm step if
the global object is an ordinary
object. However, if the
global object is a Proxy exotic object it may
exhibit behaviours that cause abnormal terminations in some of the following steps.
12.
NOTE: Annex B.3.2.2
adds additional steps at this point.
c. Perform ? env.CreateGlobalFunctionBinding(fn,
fo, false).
17. For each String vn of declaredVarNames, do
a. Perform ? env.CreateGlobalVarBinding(vn,
false).
18. Return unused.
Note 2
Early
errors specified in 16.1.1
prevent name conflicts between function/var declarations and let/const/class declarations as well as
redeclaration of let/const/class bindings for declaration contained within a single Script. However, such
conflicts and redeclarations that span more than one Script are detected as runtime errors
during GlobalDeclarationInstantiation. If any such errors are detected, no bindings are instantiated for
the script. However, if the global object is defined using
Proxy exotic objects then
the runtime tests for conflicting declarations may be unreliable resulting in an abrupt
completion and some global declarations not being instantiated. If this occurs, the
code for the Script
is not evaluated.
Unlike explicit var or function declarations, properties that are directly created on the global
object result in global
bindings that may be shadowed by let/const/class declarations.
The duplicate ExportedNames
rule implies that multiple export defaultExportDeclaration items
within a ModuleBody is a Syntax Error.
Additional error conditions relating to conflicting or duplicate declarations are checked during
module linking prior to evaluation of a Module. If any such errors are
detected the Module is not evaluated.
The abstract operation ImportedLocalNames takes argument importEntries (a List of
ImportEntry Records) and
returns a List of
Strings. It creates a List of all
of the local name bindings defined by importEntries. It performs the following steps when
called:
A Module Record encapsulates structural information
about the imports and exports of a single module. This information is used to link the imports and exports
of sets of connected modules. A Module Record includes four fields that are only used when evaluating a
module.
For specification purposes Module Record values are values of the Record
specification type and can be thought of as existing in a simple object-oriented hierarchy where Module
Record is an abstract class with both abstract and concrete subclasses. This specification defines the
abstract subclass named Cyclic Module Record and its
concrete subclass named Source Text Module Record.
Other specifications and implementations may define additional Module Record subclasses corresponding to
alternative module definition facilities that they defined.
Module Record defines the fields listed in Table 41. All Module
Definition subclasses include at least those fields. Module Record also defines the abstract method list
in Table 42.
All Module definition subclasses must provide concrete implementations of these abstract methods.
Prepares the module for linking by recursively loading all its dependencies, and returns a
promise.
GetExportedNames([exportStarSet])
Return a list of all names that are either directly or indirectly exported from this module.
LoadRequestedModules must have completed successfully prior to invoking this method.
ResolveExport(exportName [, resolveSet])
Return the binding of a name exported by this module. Bindings are represented by a ResolvedBinding
Record, of the form { [[Module]]: Module
Record, [[BindingName]]: String |
namespace }. If the export is a Module Namespace Object without a
direct binding in any module, [[BindingName]] will be set to
namespace. Return null if the name cannot be
resolved, or ambiguous if multiple bindings were found.
Each time this operation is called with a specific exportName,
resolveSet pair as arguments it must return the same result.
LoadRequestedModules must have completed successfully prior to invoking this method.
Link()
Prepare the module for evaluation by transitively resolving all module dependencies and
creating a Module Environment
Record.
LoadRequestedModules must have completed successfully prior to invoking this method.
Evaluate()
Returns a promise for the evaluation of this module and its dependencies, resolving on
successful evaluation or if it has already been evaluated successfully, and rejecting for an
evaluation error or if it has already been evaluated unsuccessfully. If the promise is
rejected, hosts are expected to handle the
promise rejection and rethrow the evaluation error.
Link must have completed successfully prior to invoking this method.
16.2.1.5 Cyclic Module Records
A Cyclic Module
Record is used to represent information about a module that can participate in dependency cycles
with other modules that are subclasses of the Cyclic Module Record type.
Module Records that
are not subclasses of the Cyclic Module Record type
must not participate in dependency cycles with Source Text Module Records.
new, unlinked, linking,
linked, evaluating,
evaluating-async, or evaluated
Initially new. Transitions to unlinked,
linking, linked,
evaluating, possibly evaluating-async,
evaluated (in that order) as the module progresses throughout its
lifecycle. evaluating-async indicates this module is queued to execute on
completion of its asynchronous dependencies or it is a module whose [[HasTLA]] field is true that has been executed and is
pending top-level completion.
Auxiliary field used during Link and Evaluate only. If [[Status]] is
either linking or evaluating, this non-negative
number records the point at which the module was first visited during the depth-first traversal
of the dependency graph.
Auxiliary field used during Link and Evaluate only. If [[Status]] is
either linking or evaluating, this is either the
module's own [[DFSIndex]] or that of an "earlier" module in the same
strongly connected component.
A List
of all the ModuleSpecifier
strings used by the module represented by this record to request the importation of a module.
The List
is in source text occurrence order.
A map from the specifier strings used by the module represented by this record to request the
importation of a module to the resolved Module
Record. The list does not contain two different Records
with the same [[Specifier]].
The first visited module of the cycle, the root DFS ancestor of the strongly connected
component. For a module not in a cycle, this would be the module itself. Once Evaluate has
completed, a module's [[DFSAncestorIndex]] is the [[DFSIndex]] of its [[CycleRoot]].
[[HasTLA]]
a Boolean
Whether this module is individually asynchronous (for example, if it's a Source Text Module
Record containing a top-level await). Having an asynchronous dependency does
not mean this field is true. This field must not change after the module is
parsed.
[[AsyncEvaluation]]
a Boolean
Whether this module is either itself asynchronous or has an asynchronous dependency. Note: The
order in which this field is set is used to order queued executions, see 16.2.1.5.3.4.
If this module is the [[CycleRoot]] of some cycle, and Evaluate() was
called on some module in that cycle, this field contains the PromiseCapability
Record for that entire evaluation. It is used to settle the Promise object
that is returned from the Evaluate() abstract method. This field will be
empty for any dependencies of that module, unless a top-level Evaluate()
has been initiated for some of those dependencies.
If this module or a dependency has [[HasTLA]]true,
and execution is in progress, this tracks the parent importers of this module for the top-level
execution job. These parent modules will not start executing before this module has successfully
completed execution.
If this module has any asynchronous dependencies, this tracks the number of asynchronous
dependency modules remaining to execute for this module. A module with asynchronous dependencies
will be executed when this field reaches 0 and there are no execution errors.
Evaluate the module's code within its execution
context. If this module has true in [[HasTLA]], then a PromiseCapability
Record is passed as an argument, and the method is expected to resolve or
reject the given capability. In this case, the method must not throw an exception, but instead
reject the PromiseCapability
Record if necessary.
A GraphLoadingState
Record is a Record that
contains information about the loading process of a module graph. It's used to continue loading after a
call to HostLoadImportedModule.
Each GraphLoadingState Record
has the fields defined in Table 45:
It is a list of the Cyclic Module
Records that have been already loaded by the current loading process, to
avoid infinite loops with circular dependencies.
The LoadRequestedModules concrete method of a Cyclic Module Recordmodule takes optional argument hostDefined (anything) and returns a Promise. It
populates the [[LoadedModules]] of all the Module Records in
the dependency graph of module (most of the work is done by the auxiliary function InnerModuleLoading). It
takes an optional hostDefined parameter that is passed to the HostLoadImportedModule
hook. It performs the following steps when called:
1. If hostDefined is not present, let
hostDefined be empty.
3. Let state be the GraphLoadingState
Record { [[IsLoading]]: true, [[PendingModulesCount]]: 1, [[Visited]]: « »,
[[PromiseCapability]]: pc, [[HostDefined]]: hostDefined }.
The hostDefined parameter can be used to pass additional information necessary to fetch the
imported modules. It is used, for example, by HTML to set the correct fetch destination for
<link rel="preload" as="..."> tags.
import() expressions never set the hostDefined parameter.
16.2.1.5.1.1 InnerModuleLoading ( state, module )
The abstract operation InnerModuleLoading takes arguments state (a GraphLoadingState
Record) and module (a Module
Record) and returns unused. It is used by
LoadRequestedModules to recursively perform the actual loading process for module's
dependency graph. It performs the following steps when called:
b. Perform ! Call(state.[[PromiseCapability]].[[Reject]],
undefined, « moduleCompletion.[[Value]] »).
4. Return unused.
16.2.1.5.2 Link ( )
The Link concrete method of a Cyclic Module Recordmodule takes no arguments and returns either a normal completion
containingunused or a throw
completion. On success, Link transitions this module's [[Status]] from unlinked to linked.
On failure, an exception is thrown and this module's [[Status]] remains
unlinked. (Most of the work is done by the auxiliary function InnerModuleLinking.) It
performs the following steps when called:
1. Assert: module.[[Status]] is one of unlinked,
linked, evaluating-async, or
evaluated.
16.2.1.5.2.1 InnerModuleLinking ( module, stack,
index )
The abstract operation InnerModuleLinking takes arguments module (a Module Record),
stack (a List of
Cyclic Module Records),
and index (a non-negative integer) and returns either a
normal completion
containing a non-negative integer or a throw
completion. It is used by Link to perform the actual linking process for
module, as well as recursively on all other modules in the dependency graph. The
stack and index parameters, as well as a module's [[DFSIndex]] and [[DFSAncestorIndex]] fields, keep
track of the depth-first search (DFS) traversal. In particular, [[DFSAncestorIndex]] is used to discover strongly connected components (SCCs),
such that all modules in an SCC transition to linked together. It performs the
following steps when called:
v. If requiredModule and module
are the same Module
Record, set done to true.
14. Return index.
16.2.1.5.3 Evaluate ( )
The Evaluate concrete method of a Cyclic Module Recordmodule takes no arguments and returns a Promise. Evaluate transitions this module's [[Status]] from linked to either
evaluating-async or evaluated. The first time it is called
on a module in a given strongly connected component, Evaluate creates and returns a Promise which
resolves when the module has finished evaluating. This Promise is stored in the [[TopLevelCapability]] field of the [[CycleRoot]] for the
component. Future invocations of Evaluate on any module in the component return the same Promise. (Most
of the work is done by the auxiliary function InnerModuleEvaluation.)
It performs the following steps when called:
1. Assert: This call to Evaluate is not
happening at the same time as another call to Evaluate within the surrounding agent.
2. Assert: module.[[Status]] is one of linked,
evaluating-async, or evaluated.
3. If module.[[Status]] is
either evaluating-async or evaluated, set
module to module.[[CycleRoot]].
4. If module.[[TopLevelCapability]] is not empty, then
a. Return module.[[TopLevelCapability]].[[Promise]].
16.2.1.5.3.1 InnerModuleEvaluation ( module,
stack, index )
The abstract operation InnerModuleEvaluation takes arguments module (a Module Record),
stack (a List of
Cyclic Module Records),
and index (a non-negative integer) and returns either a
normal completion
containing a non-negative integer or a throw
completion. It is used by Evaluate to perform the actual evaluation process for
module, as well as recursively on all other modules in the dependency graph. The
stack and index parameters, as well as module's [[DFSIndex]] and [[DFSAncestorIndex]] fields, are used
the same way as in InnerModuleLinking. It
performs the following steps when called:
iv. If requiredModule.[[AsyncEvaluation]] is false, set
requiredModule.[[Status]] to
evaluated.
v. Otherwise, set requiredModule.[[Status]] to evaluating-async.
vi. If requiredModule and module
are the same Module
Record, set done to true.
vii. Set requiredModule.[[CycleRoot]] to module.
17. Return index.
Note 1
A module is evaluating while it is being traversed by
InnerModuleEvaluation. A module is evaluated on execution completion or
evaluating-async during execution if its [[HasTLA]] field is true or if it has asynchronous
dependencies.
Note 2
Any modules depending on a module of an asynchronous cycle when that cycle is not
evaluating will instead depend on the execution of the root of the cycle
via [[CycleRoot]]. This ensures that the cycle state can be treated as a
single strongly connected component through its root module state.
16.2.1.5.3.2 ExecuteAsyncModule ( module )
The abstract operation ExecuteAsyncModule takes argument module (a Cyclic Module Record) and
returns unused. It performs the following steps when called:
1. Assert: module.[[Status]] is either evaluating or
evaluating-async.
The abstract operation GatherAvailableAncestors takes arguments module (a Cyclic Module Record) and
execList (a List of
Cyclic Module Records)
and returns unused. It performs the following steps when called:
When an asynchronous execution for a root module is fulfilled, this function
determines the list of modules which are able to synchronously execute together on this
completion, populating them in execList.
The abstract operation AsyncModuleExecutionFulfilled takes argument module (a Cyclic Module Record) and
returns unused. It performs the following steps when called:
1. If module.[[Status]] is
evaluated, then
a. Assert: module.[[EvaluationError]] is not empty.
10. Let sortedExecList be a List
whose elements are the elements of execList, in the order in which they had their [[AsyncEvaluation]] fields set to true in InnerModuleEvaluation.
11. Assert: All elements of
sortedExecList have their [[AsyncEvaluation]] field set to
true, [[PendingAsyncDependencies]] field set to 0, and
[[EvaluationError]] field set to empty.
The abstract operation AsyncModuleExecutionRejected takes arguments module (a Cyclic Module Record) and
error (an ECMAScript language
value) and returns unused. It performs the following steps
when called:
1. If module.[[Status]] is
evaluated, then
a. Assert: module.[[EvaluationError]] is not empty.
b. Perform ! Call(module.[[TopLevelCapability]].[[Reject]],
undefined, « error »).
9. Return unused.
16.2.1.5.4 Example Cyclic Module Record Graphs
This non-normative section gives a series of examples of the linking and evaluation of a few common
module graphs, with a specific focus on how errors can occur.
First consider the following simple module graph:
Figure 2: A simple module graph
Let's first assume that there are no error conditions. When a host first calls
A.LoadRequestedModules(), this will complete successfully by assumption, and recursively load
the dependencies of B and C as well (respectively, C and none), and
then set A.[[Status]] = B.[[Status]] = C.[[Status]] =
unlinked. Then, when the host calls A.Link(), it will
complete successfully (again by assumption) such that A.[[Status]] =
B.[[Status]] = C.[[Status]] =
linked. These preparatory steps can be performed at any time. Later, when the host is ready to incur any
possible side effects of the modules, it can call A.Evaluate(), which will complete
successfully, returning a Promise resolving to undefined (again by assumption),
recursively having evaluated first C and then B. Each module's [[Status]] at this point will be evaluated.
Consider then cases involving linking errors, after a successful call to
A.LoadRequestedModules(). If InnerModuleLinking of
C succeeds but, thereafter, fails for B, for example because it imports something
that C does not provide, then the original A.Link() will fail, and both
A and B's [[Status]] remain
unlinked. C's [[Status]] has become
linked, though.
Finally, consider a case involving evaluation errors after a successful call to Link(). If InnerModuleEvaluation
of C succeeds but, thereafter, fails for B, for example because B
contains code that throws an exception, then the original A.Evaluate() will fail, returning a
rejected Promise. The resulting exception will be recorded in both A and B's [[EvaluationError]] fields, and their [[Status]] will
become evaluated. C will also become evaluated
but, in contrast to A and B, will remain without an [[EvaluationError]], as it successfully completed evaluation. Storing the
exception ensures that any time a host tries to reuse A or
B by calling their Evaluate() method, it will encounter the same exception. (Hosts are not
required to reuse Cyclic Module Records;
similarly, hosts are not required to expose the
exception objects thrown by these methods. However, the specification enables such uses.)
Now consider a different type of error condition:
Figure 3: A module graph with an unresolvable module
In this scenario, module A declares a dependency on some other module, but no Module Record exists
for that module, i.e. HostLoadImportedModule
calls FinishLoadingImportedModule
with an exception when asked for it. This could occur for a variety of reasons, such as the
corresponding resource not existing, or the resource existing but ParseModule
returning some errors when trying to parse the resulting source text. Hosts can choose to expose
the cause of failure via the completion they pass to FinishLoadingImportedModule.
In any case, this exception causes a loading failure, which results in A's [[Status]] remaining new.
The difference here between loading, linking and evaluation errors is due to the following
characteristic:
Evaluation must be only
performed once, as it can cause side effects; it is thus important to remember whether evaluation has
already been performed, even if unsuccessfully. (In the error case, it makes sense to also remember
the exception because otherwise subsequent Evaluate() calls would have to synthesize a new one.)
Linking, on the other hand, is side-effect-free, and thus even if it fails, it can be retried at a
later time with no issues.
Loading closely interacts with the host, and it may be desiderable for some
of them to allow users to retry failed loads (for example, if the failure is caused by temporarily bad
network conditions).
Now, consider a module graph with a cycle:
Figure 4: A cyclic module graph
Here we assume that the entry point is module A, so that the host proceeds by calling
A.LoadRequestedModules(), which performs InnerModuleLoading on
A. This in turn calls InnerModuleLoading on
B and C. Because of the cycle, this again triggers InnerModuleLoading on
A, but at this point it is a no-op since A's dependencies loading has already been
triggered during this LoadRequestedModules process. When all the modules in the graph have been
successfully loaded, their [[Status]] transitions from
new to unlinked at the same time.
Then the host proceeds by calling
A.Link(), which performs InnerModuleLinking on
A. This in turn calls InnerModuleLinking on
B. Because of the cycle, this again triggers InnerModuleLinking on
A, but at this point it is a no-op since A.[[Status]] is
already linking. B.[[Status]] itself remains
linking when control gets back to A and InnerModuleLinking is
triggered on C. After this returns with C.[[Status]]
being linked, both A and B transition from
linking to linked together; this is by design, since they
form a strongly connected component. It's possible to transition the status of modules in the same SCC
at the same time because during this phase the module graph is traversed with a depth-first search.
An analogous story occurs for the evaluation phase of a cyclic module graph, in the success case.
Now consider a case where A has a linking error; for example, it tries to import a binding
from C that does not exist. In that case, the above steps still occur, including the early
return from the second call to InnerModuleLinking on
A. However, once we unwind back to the original InnerModuleLinking on
A, it fails during InitializeEnvironment, namely right after C.ResolveExport().
The thrown SyntaxError exception propagates up to A.Link, which resets all
modules that are currently on its stack (these are always exactly the modules that are still
linking). Hence both A and B become
unlinked. Note that C is left as linked.
Alternatively, consider a case where A has an evaluation error; for example, its source code
throws an exception. In that case, the evaluation-time analog of the above steps still occurs, including
the early return from the second call to InnerModuleEvaluation
on A. However, once we unwind back to the original InnerModuleEvaluation
on A, it fails by assumption. The exception thrown propagates up to A.Evaluate(),
which records the error in all modules that are currently on its stack (i.e., the modules
that are still evaluating) as well as via [[AsyncParentModules]], which form a chain for modules which contain or depend on
top-level await through the whole dependency graph through the AsyncModuleExecutionRejected
algorithm. Hence both A and B become evaluated and the
exception is recorded in both A and B's [[EvaluationError]] fields, while C is left as
evaluated with no [[EvaluationError]].
Lastly, consider a module graph with a cycle, where all modules complete asynchronously:
Figure 5: An asynchronous cyclic module graph
Loading and linking happen as before, and all modules end up with [[Status]]
set to linked.
Calling A.Evaluate() calls InnerModuleEvaluation
on A, B, and D, which all transition to
evaluating. Then InnerModuleEvaluation
is called on A again, which is a no-op because it is already
evaluating. At this point, D.[[PendingAsyncDependencies]] is 0, so ExecuteAsyncModule(D)
is called and we call D.ExecuteModule with a new PromiseCapability tracking the asynchronous
execution of D. We unwind back to the InnerModuleEvaluation
on B, setting B.[[PendingAsyncDependencies]] to 1 and
B.[[AsyncEvaluation]] to true. We unwind back to
the original InnerModuleEvaluation
on A, setting A.[[PendingAsyncDependencies]] to 1. In the
next iteration of the loop over A's dependencies, we call InnerModuleEvaluation
on C and thus on D (again a no-op) and E. As E has no
dependencies and is not part of a cycle, we call ExecuteAsyncModule(E)
in the same manner as D and E is immediately removed from the stack. We unwind
once more to the original InnerModuleEvaluation
on A, setting C.[[AsyncEvaluation]] to
true. Now we finish the loop over A's dependencies, set A.[[AsyncEvaluation]] to true, and remove the entire strongly
connected component from the stack, transitioning all of the modules to
evaluating-async at once. At this point, the fields of the modules are as given
in Table
46.
Table 46: Module fields after the initial Evaluate() call
Module
[[DFSIndex]]
[[DFSAncestorIndex]]
[[Status]]
[[AsyncEvaluation]]
[[AsyncParentModules]]
[[PendingAsyncDependencies]]
A
0
0
evaluating-async
true
« »
2 (B and C)
B
1
0
evaluating-async
true
« A »
1 (D)
C
2
0
evaluating-async
true
« A »
2 (D and E)
D
3
0
evaluating-async
true
« B, C »
0
E
4
4
evaluating-async
true
« C »
0
Let us assume that E finishes executing first. When that happens, AsyncModuleExecutionFulfilled
is called, E.[[Status]] is set to evaluated
and C.[[PendingAsyncDependencies]] is decremented to become 1. The
fields of the updated modules are as given in Table
47.
Table 47: Module fields after module E finishes executing
Module
[[DFSIndex]]
[[DFSAncestorIndex]]
[[Status]]
[[AsyncEvaluation]]
[[AsyncParentModules]]
[[PendingAsyncDependencies]]
C
2
0
evaluating-async
true
« A »
1 (D)
E
4
4
evaluated
true
« C »
0
D is next to finish (as it was the only module that was still executing). When that happens,
AsyncModuleExecutionFulfilled
is called again and D.[[Status]] is set to
evaluated. Then B.[[PendingAsyncDependencies]] is decremented to become 0, ExecuteAsyncModule is
called on B, and it starts executing. C.[[PendingAsyncDependencies]] is also decremented to become 0, and C
starts executing (potentially in parallel to B if B contains an
await). The fields of the updated modules are as given in Table
48.
Table 48: Module fields after module D finishes executing
Module
[[DFSIndex]]
[[DFSAncestorIndex]]
[[Status]]
[[AsyncEvaluation]]
[[AsyncParentModules]]
[[PendingAsyncDependencies]]
B
1
0
evaluating-async
true
« A »
0
C
2
0
evaluating-async
true
« A »
0
D
3
0
evaluated
true
« B, C »
0
Let us assume that C finishes executing next. When that happens, AsyncModuleExecutionFulfilled
is called again, C.[[Status]] is set to
evaluated and A.[[PendingAsyncDependencies]]
is decremented to become 1. The fields of the updated modules are as given in Table
49.
Table 49: Module fields after module C finishes executing
Module
[[DFSIndex]]
[[DFSAncestorIndex]]
[[Status]]
[[AsyncEvaluation]]
[[AsyncParentModules]]
[[PendingAsyncDependencies]]
A
0
0
evaluating-async
true
« »
1 (B)
C
2
0
evaluated
true
« A »
0
Then, B finishes executing. When that happens, AsyncModuleExecutionFulfilled
is called again and B.[[Status]] is set to
evaluated. A.[[PendingAsyncDependencies]] is
decremented to become 0, so ExecuteAsyncModule is
called and it starts executing. The fields of the updated modules are as given in Table
50.
Table 50: Module fields after module B finishes executing
Module
[[DFSIndex]]
[[DFSAncestorIndex]]
[[Status]]
[[AsyncEvaluation]]
[[AsyncParentModules]]
[[PendingAsyncDependencies]]
A
0
0
evaluating-async
true
« »
0
B
1
0
evaluated
true
« A »
0
Finally, A finishes executing. When that happens, AsyncModuleExecutionFulfilled
is called again and A.[[Status]] is set to
evaluated. At this point, the Promise in A.[[TopLevelCapability]] (which was returned from A.Evaluate()) is
resolved, and this concludes the handling of this module graph. The fields of the updated module are as
given in Table
51.
Table 51: Module fields after module A finishes executing
Module
[[DFSIndex]]
[[DFSAncestorIndex]]
[[Status]]
[[AsyncEvaluation]]
[[AsyncParentModules]]
[[PendingAsyncDependencies]]
A
0
0
evaluated
true
« »
0
Alternatively, consider a failure case where C fails execution and returns an error before
B has finished executing. When that happens, AsyncModuleExecutionRejected
is called, which sets C.[[Status]] to
evaluated and C.[[EvaluationError]] to the
error. It then propagates this error to all of the AsyncParentModules by performing AsyncModuleExecutionRejected
on each of them. The fields of the updated modules are as given in Table
52.
Table 52: Module fields after module C finishes with an error
Module
[[DFSIndex]]
[[DFSAncestorIndex]]
[[Status]]
[[AsyncEvaluation]]
[[AsyncParentModules]]
[[PendingAsyncDependencies]]
[[EvaluationError]]
A
0
0
evaluated
true
« »
1 (B)
empty
C
2
1
evaluated
true
« A »
0
C's evaluation error
A will be rejected with the same error as C since C will call
AsyncModuleExecutionRejected
on A with C's error. A.[[Status]] is set to
evaluated. At this point the Promise in A.[[TopLevelCapability]] (which was returned from A.Evaluate()) is
rejected. The fields of the updated module are as given in Table
53.
Table 53: Module fields after module A is rejected
Then, B finishes executing without an error. When that happens, AsyncModuleExecutionFulfilled
is called again and B.[[Status]] is set to
evaluated. GatherAvailableAncestors
is called on B. However, A.[[CycleRoot]] is A
which has an evaluation error, so it will not be added to the returned sortedExecList and
AsyncModuleExecutionFulfilled
will return without further processing. Any future importer of B will resolve the rejection
of B.[[CycleRoot]].[[EvaluationError]] from
the evaluation error from C that was set on the cycle root A. The fields of the
updated modules are as given in Table
54.
Table 54: Module fields after module B finishes executing in an erroring graph
A Source Text Module
Record is used to represent information about a module that was defined from ECMAScript
source text (11) that was
parsed using the goal symbolModule. Its fields
contain digested information about the names that are imported and exported by the module, and its
concrete methods use these digests to link and evaluate the module.
A List
of ExportEntry records derived from the code of this module that correspond to reexported
imports that occur within the module or exports from export * as namespace
declarations.
A List
of ExportEntry records derived from the code of this module that correspond to
export * declarations that occur within the module, not including
export * as namespace declarations.
An ImportEntry Record is
a Record that
digests information about a single declarative import. Each ImportEntry
Record has the fields defined in Table
56:
The name under which the desired binding is exported by the module identified by [[ModuleRequest]]. The value namespace-object
indicates that the import request is for the target module's namespace object.
[[LocalName]]
a String
The name that is used to locally access the imported value from within the importing module.
Note 1
Table
57 gives examples of ImportEntry records fields used to represent the syntactic
import forms:
An ExportEntry Record is
a Record that
digests information about a single declarative export. Each ExportEntry
Record has the fields defined in Table 58:
The name under which the desired binding is exported by the module identified by [[ModuleRequest]]. null if the ExportDeclaration
does not have a ModuleSpecifier.
all is used for export * as ns from "mod" declarations.
all-but-default is used for export * from "mod"
declarations.
[[LocalName]]
a String or null
The name that is used to locally access the exported value from within the importing module.
null if the exported value is not locally accessible from within the module.
Note 2
Table
59 gives examples of the ExportEntry record fields used to represent the syntactic
export forms:
i. If importedBoundNames does not contain
ee.[[LocalName]], then
1. Append ee to
localExportEntries.
ii. Else,
1. Let ie be the element of
importEntries whose [[LocalName]] is
ee.[[LocalName]].
2. If ie.[[ImportName]] is namespace-object, then
a. NOTE: This is a re-export of an imported module
namespace object.
b. Append ee to
localExportEntries.
3. Else,
a. NOTE: This is a re-export of a single name.
b. Append the ExportEntry
Record { [[ModuleRequest]]:
ie.[[ModuleRequest]], [[ImportName]]: ie.[[ImportName]], [[LocalName]]:
null, [[ExportName]]: ee.[[ExportName]] } to indirectExportEntries.
b. Else if ee.[[ImportName]] is all-but-default, then
An implementation may parse module source text and analyse it for Early Error conditions prior to
the evaluation of ParseModule for that module source text. However, the reporting of any errors must
be deferred until the point where this specification actually performs ParseModule upon that source
text.
The ResolveExport concrete method of a Source Text Module Recordmodule takes argument exportName (a String) and optional argument
resolveSet (a List of
Records
with fields [[Module]] (a Module
Record) and [[ExportName]] (a String)) and returns a
ResolvedBinding Record,
null, or ambiguous.
ResolveExport attempts to resolve an imported binding to the actual defining module and local binding
name. The defining module may be the module represented by the Module Record this
method was invoked on or some other module that is imported by that module. The parameter
resolveSet is used to detect unresolved circular import/export paths. If a pair consisting of
specific Module Record and
exportName is reached that is already in resolveSet, an import circularity has
been encountered. Before recursively calling ResolveExport, a pair consisting of module and
exportName is added to resolveSet.
If a defining module is found, a ResolvedBinding Record {
[[Module]], [[BindingName]] } is returned. This record
identifies the resolved binding of the originally requested export, unless this is the export of a
namespace with no local binding. In this case, [[BindingName]] will be set to
namespace. If no definition was found or the request is found to be circular,
null is returned. If the request is found to be ambiguous,
ambiguous is returned.
1. Assert: There is more
than one * import that includes the requested name.
2. If resolution.[[Module]] and starResolution.[[Module]] are not the same Module
Record, return ambiguous.
3. If resolution.[[BindingName]] is not starResolution.[[BindingName]] and either resolution.[[BindingName]] or starResolution.[[BindingName]] is namespace, return
ambiguous.
4. If resolution.[[BindingName]]is a
String, starResolution.[[BindingName]]is a
String, and SameValue(resolution.[[BindingName]], starResolution.[[BindingName]]) is false, return
ambiguous.
The abstract operation GetImportedModule takes arguments referrer (a Cyclic Module Record) and
specifier (a String) and returns a Module
Record. It performs the following steps when called:
1. Assert: Exactly one element of
referrer.[[LoadedModules]] is a Record
whose [[Specifier]] is specifier, since LoadRequestedModules has
completed successfully on referrer prior to invoking this abstract operation.
2. Let record be the Record
in referrer.[[LoadedModules]] whose [[Specifier]] is specifier.
If this operation is called multiple times with the same (referrer, specifier)
pair and it performs FinishLoadingImportedModule(referrer,
specifier, payload, result) where result is a normal
completion, then it must perform FinishLoadingImportedModule(referrer,
specifier, payload, result) with the same result each time.
The actual process performed is host-defined, but typically consists
of performing whatever I/O operations are necessary to load the appropriate Module Record.
Multiple different (referrer, specifier) pairs may map to the same Module Record
instance. The actual mapping semantics is host-defined but typically a
normalization process is applied to specifier as part of the mapping process. A typical
normalization process would include actions such as expansion of relative and abbreviated path specifiers.
16.2.1.9 FinishLoadingImportedModule ( referrer,
specifier, payload, result )
The abstract operation GetModuleNamespace takes argument module (an instance of a concrete
subclass of Module Record) and
returns a Module Namespace Object or empty. It retrieves the Module Namespace
Object representing module's exports, lazily creating it the first time it was requested, and
storing it in module.[[Namespace]] for future retrieval. It performs
the following steps when called:
GetModuleNamespace never throws. Instead, unresolvable names are simply excluded from the namespace
at this point. They will lead to a real linking error later unless they are all ambiguous star exports
that are not explicitly requested anywhere.
3. Return a List
whose sole element is a new ExportEntry Record { [[ModuleRequest]]: null, [[ImportName]]: null, [[LocalName]]:
localName, [[ExportName]]: "default" }.
3. Return a List
whose sole element is a new ExportEntry Record { [[ModuleRequest]]: null, [[ImportName]]: null, [[LocalName]]:
localName, [[ExportName]]: "default" }.
1. Let entry be the ExportEntry
Record { [[ModuleRequest]]: null, [[ImportName]]: null, [[LocalName]]:
"*default*", [[ExportName]]:
"default" }.
2. Return « entry ».
Note
"*default*" is used within this specification as a synthetic name for anonymous
default export values. See this note for more
details.
16.2.3.5 Static Semantics: ExportEntriesForModule
The syntax-directed
operation ExportEntriesForModule takes argument module (a String or
null) and returns a List of
ExportEntry Records. It is
defined piecewise over the following productions:
4. Return a List
whose sole element is a new ExportEntry Record { [[ModuleRequest]]: module, [[ImportName]]:
importName, [[LocalName]]: localName, [[ExportName]]: sourceName }.
5. Return a List
whose sole element is a new ExportEntry Record { [[ModuleRequest]]: module, [[ImportName]]:
importName, [[LocalName]]: localName, [[ExportName]]: exportName }.
An implementation must report most errors at the time the relevant ECMAScript language construct is evaluated.
An early error is an error that can be
detected and reported prior to the evaluation of any construct in the Script containing the error. The presence of
an early
error prevents the evaluation of the construct. An implementation must report early
errors in a Script as part of parsing that Script in ParseScript. Early
errors in a Module are reported at the point when the
Module would be
evaluated and the Module is
never initialized. Early
errors in eval code are reported at the time eval is called and prevent
evaluation of the eval code. All errors that are not early errors are runtime errors.
An implementation must report as an early error any occurrence of a condition
that is listed in a “Static Semantics: Early Errors” subclause of this specification.
An implementation shall not treat other kinds of errors as early errors even if the compiler can prove
that a construct cannot execute without error under any circumstances. An implementation may issue an early
warning in such a case, but it should not report the error until the relevant construct is actually executed.
An implementation shall report all errors as specified, except for the following:
Except as restricted in 17.1, a host or
implementation may extend Script syntax, Module syntax, and regular expression
pattern or flag syntax. To permit this, all operations (such as calling eval, using a regular
expression literal, or using the Function or RegExp constructor) that are allowed to throw
SyntaxError are permitted to exhibit host-defined behaviour instead of
throwing SyntaxError when they encounter a host-defined extension to the script
syntax or regular expression pattern or flag syntax.
Except as restricted in 17.1, a host or
implementation may provide additional types, values, objects, properties, and functions beyond those described
in this specification. This may cause constructs (such as looking up a variable in the global scope) to have
host-defined behaviour instead of
throwing an error (such as ReferenceError).
17.1 Forbidden Extensions
An implementation must not extend this specification in the following ways:
If an implementation extends any function object with an own property
named "caller" the value of that property, as observed using [[Get]] or [[GetOwnProperty]], must not be a strict
function object. If it is an accessor property, the function that
is the value of the property's [[Get]] attribute must never return a strict
function when called.
Neither mapped nor unmapped arguments objects may be created with an own property named
"caller".
The behaviour of built-in methods which are specified in ECMA-402, such as those named
toLocaleString, must not be extended except as specified in ECMA-402.
The RegExp pattern grammars in 22.2.1 and B.1.2 must not be
extended to recognize any of the source characters A-Z or a-z as IdentityEscape[+UnicodeMode]
when the [UnicodeMode] grammar parameter is present.
The Syntactic Grammar must not be extended in any manner that allows the token : to immediately
follow source text that is matched by the BindingIdentifier nonterminal
symbol.
There are certain built-in objects available whenever an ECMAScript Script or Module begins execution. One, the global
object, is part of the global environment of the executing program. Others are accessible as
initial properties of the global object or indirectly as
properties of accessible built-in objects.
Unless specified otherwise, a built-in object that is callable as a function is a built-in function
object with the characteristics described in 10.3.
Unless specified otherwise, the [[Extensible]] internal slot of a built-in object
initially has the value true. Every built-in function object has a
[[Realm]] internal slot whose value is the Realm Record of the
realm for
which the object was initially created.
Many built-in objects are functions: they can be invoked with arguments. Some of them furthermore are constructors: they are functions intended
for use with the new operator. For each built-in function, this specification describes the
arguments required by that function and the properties of that function object. For
each built-in constructor, this specification furthermore
describes properties of the prototype object of that constructor and properties of specific
object instances returned by a new expression that invokes that constructor.
Unless otherwise specified in the description of a particular function, if a built-in function or constructor
is given fewer arguments than
the function is specified to require, the function or constructor shall behave exactly as if it
had been given sufficient additional arguments, each such argument being the undefined value.
Such missing arguments are considered to be “not present” and may be identified in that manner by specification
algorithms. In the description of a particular function, the terms “this value” and
“NewTarget” have the meanings given in 10.3.
Unless otherwise specified in the description of a particular function, if a built-in function or constructor
described is given more
arguments than the function is specified to allow, the extra arguments are evaluated by the call and then
ignored by the function. However, an implementation may define implementation specific behaviour relating to
such arguments as long as the behaviour is not the throwing of a TypeError exception that is
predicated simply on the presence of an extra argument.
Note 1
Implementations that add additional capabilities to the set of built-in functions are encouraged to do so
by adding new functions rather than adding new parameters to existing functions.
Unless otherwise specified every built-in function and every built-in constructor has the
Function prototype
object, which is the initial value of the expression Function.prototype
(20.2.3),
as the value of its [[Prototype]] internal slot.
Unless otherwise specified every built-in prototype object has the Object prototype
object, which is the initial value of the expression Object.prototype (20.1.3),
as the value of its [[Prototype]] internal slot, except the Object prototype
object itself.
If this specification defines a built-in constructor's behaviour via algorithm steps,
then that is its behaviour for the purposes of both [[Call]] and [[Construct]]. If such an algorithm needs to distinguish the two cases, it checks whether
NewTarget is undefined, which indicates a [[Call]] invocation.
Built-in function objects that are not identified
as constructors do not implement the [[Construct]] internal method unless otherwise specified in the description of a
particular function.
Built-in function objects that are not constructors
do not have a
"prototype" property unless otherwise specified in the description of a particular function.
Each built-in function defined in this specification is created by calling the CreateBuiltinFunction abstract
operation (10.3.4). The values of the
length and name parameters are the initial values of the "length" and
"name" properties as discussed below. The values of the prefix parameter are
similarly discussed below.
Every built-in function object, including constructors, has a
"length" property whose value is a non-negative integral Number. Unless
otherwise specified, this value is the number of required parameters shown in the subclause heading for the
function description. Optional parameters and rest parameters are not included in the parameter count.
Note 2
For example, the function object that is the initial
value of the "map" property of the Array prototype
object is described under the subclause heading «Array.prototype.map (callbackFn [ ,
thisArg])» which shows the two named arguments callbackFn and thisArg, the latter being optional; therefore
the value of the "length" property of that function object is
1𝔽.
Unless otherwise specified, the "length" property of a built-in function
object has the attributes { [[Writable]]: false,
[[Enumerable]]: false, [[Configurable]]:
true }.
Every built-in function object, including constructors, has a
"name" property whose value is a
String. Unless otherwise specified, this value is the name that is given to the function in
this specification. Functions that are identified as anonymous functions use the empty String as the value of
the "name" property. For functions that are specified as properties of objects, the name
value is the property name string used to access the
function. Functions that are specified as get or set accessor functions of built-in properties have
"get" or "set" (respectively) passed to the prefix parameter
when calling CreateBuiltinFunction.
The value of the "name" property is explicitly specified for each built-in functions whose
property
keyis a Symbol
value. If such an explicitly specified value starts with the prefix "get " or "set
" and the function for which it is specified is a get or set accessor function of a built-in
property, the value without the prefix is passed to the name parameter, and the value
"get" or "set" (respectively) is passed to the prefix parameter
when calling CreateBuiltinFunction.
Unless otherwise specified, the "name" property of a built-in function
object has the attributes { [[Writable]]: false,
[[Enumerable]]: false, [[Configurable]]:
true }.
Every other data property described in clauses
19 through 28
and in Annex B.2 has the
attributes { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]:
true } unless otherwise specified.
Every accessor property described in clauses
19 through 28
and in Annex B.2 has the
attributes { [[Enumerable]]: false, [[Configurable]]: true } unless otherwise specified. If only a get
accessor function is described, the set accessor function is the default value, undefined. If
only a set accessor is described the get accessor is the default value, undefined.
does not have a [[Construct]] internal method; it cannot be used as a constructor with the new
operator.
does not have a [[Call]] internal method; it cannot be invoked as a function.
has a [[Prototype]] internal slot whose value is host-defined.
may have host-defined properties in addition to
the properties defined in this specification. This may include a property whose value is the global object
itself.
19.1 Value Properties of the Global Object
19.1.1 globalThis
The initial value of the "globalThis" property of the global
object in a Realm Recordrealm is
realm.[[GlobalEnv]].[[GlobalThisValue]].
This property has the attributes { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]:
true }.
19.1.2 Infinity
The value of Infinity is +∞𝔽 (see 6.1.6.1).
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.
19.1.3 NaN
The value of NaN is NaN (see 6.1.6.1).
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.
19.1.4 undefined
The value of undefined is undefined (see 6.1.1).
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.
15. NOTE: If direct is true,
runningContext will be the execution context that
performed the direct
eval. If direct is false, runningContext
will be the execution context for
the invocation of the eval function.
The eval code cannot instantiate variable or function bindings in the variable environment of the
calling context that invoked the eval if either the code of the calling context or the eval code is
strict mode code. Instead
such bindings are instantiated in a new VariableEnvironment that is only accessible to the eval code.
Bindings introduced by let, const, or class declarations are
always instantiated in a new LexicalEnvironment.
19.2.1.2 HostEnsureCanCompileStrings ( calleeRealm,
parameterStrings, bodyString, direct )
The host-defined abstract operation
HostEnsureCanCompileStrings takes arguments calleeRealm (a Realm Record),
parameterStrings (a List of
Strings), bodyString (a String), and direct (a Boolean) and returns either a
normal completion
containingunused or a throw
completion. It allows host environments to block
certain ECMAScript functions which allow developers to interpret and evaluate strings as ECMAScript code.
parameterStrings represents the strings that, when using one of the function constructors, will be concatenated
together to build the parameters list. bodyString represents the function body or the string
passed to an eval call.
direct signifies whether the evaluation is a direct
eval.
The default implementation of HostEnsureCanCompileStrings is to return NormalCompletion(unused).
4. Let trimmedPrefix be the longest prefix of
trimmed that satisfies the syntax of a StrDecimalLiteral, which
might be trimmed itself. If there is no such prefix, return NaN.
This function may interpret only a leading portion of string as a Number value; it ignores
any code units that cannot be interpreted as part of the notation of a decimal literal, and no
indication is given that any such code units were ignored.
19.2.5 parseInt ( string, radix )
This function produces an integral Number dictated by
interpretation of the contents of string according to the specified radix. Leading
white space in string is ignored. If radix coerces to 0 (such as when it is
undefined), it is assumed to be 10 except when the number representation begins with
"0x" or "0X", in which case it is assumed to be 16. If
radix is 16, the number representation may optionally begin with "0x" or
"0X".
4. If S is not empty and the first code unit of
S is the code unit 0x002D (HYPHEN-MINUS), set sign to -1.
5. If S is not empty and the first code unit of
S is either the code unit 0x002B (PLUS SIGN) or the code unit 0x002D (HYPHEN-MINUS), set
S to the substring of S from index
1.
11. If S contains a code unit that is not a
radix-R digit, let end be the index within S of the first such code
unit; otherwise, let end be the length of S.
14. Let mathInt be the integer value that is
represented by Z in radix-R notation, using the letters A through Z
and a through z for digits with values 10 through 35. (However, if R = 10 and
Z contains more than 20 significant digits, every significant digit after the 20th may be
replaced by a 0 digit, at the option of the implementation; and if R is not one of 2, 4, 8,
10, 16, or 32, then mathInt may be an implementation-approximatedinteger representing the integer
value denoted by Z in radix-R notation.)
This function may interpret only a leading portion of string as an integer value; it
ignores any code units that cannot be interpreted as part of the notation of an integer,
and no indication is given that any such code units were ignored.
19.2.6 URI Handling Functions
Uniform Resource Identifiers, or URIs, are Strings that identify resources (e.g. web pages or files) and
transport protocols by which to access them (e.g. HTTP or FTP) on the Internet. The ECMAScript language
itself does not provide any support for using URIs except for functions that encode and decode URIs as
described in this section. encodeURI and decodeURI are intended to work with
complete URIs; they assume that any reserved characters are intended to have special meaning (e.g., as
delimiters) and so are not encoded. encodeURIComponent and decodeURIComponent are
intended to work with the individual components of a URI; they assume that any reserved characters represent
text and must be encoded to avoid special meaning when the component is part of a complete URI.
Note 1
The set of reserved characters is based upon RFC 2396 and does not reflect changes introduced by the
more recent RFC 3986.
Note 2
Many implementations of ECMAScript provide additional functions and methods that manipulate web pages;
these functions are beyond the scope of this standard.
19.2.6.1 decodeURI ( encodedURI )
This function computes a new version of a URI in which each escape sequence and UTF-8 encoding of the
sort that might be introduced by the encodeURI function is replaced with the UTF-16 encoding
of the code point that it represents. Escape sequences that could not have been introduced by
encodeURI are not replaced.
This function computes a new version of a URI in which each escape sequence and UTF-8 encoding of the
sort that might be introduced by the encodeURIComponent function is replaced with the UTF-16
encoding of the code point that it represents.
It is the %decodeURIComponent% intrinsic object.
It performs the following steps when called:
1. Let componentString be ? ToString(encodedURIComponent).
This function computes a new version of a UTF-16 encoded (6.1.4)
URI in which each instance of certain code points is replaced by one, two, three, or four escape sequences
representing the UTF-8 encoding of the code point.
This function computes a new version of a UTF-16 encoded (6.1.4)
URI in which each instance of certain code points is replaced by one, two, three, or four escape sequences
representing the UTF-8 encoding of the code point.
It is the %encodeURIComponent% intrinsic object.
It performs the following steps when called:
1. Let componentString be ? ToString(uriComponent).
The abstract operation Encode takes arguments string (a String) and extraUnescaped
(a String) and returns either a normal
completion containing a String or a throw
completion. It performs URI encoding and escaping, interpreting string as a
sequence of UTF-16 encoded code points as described in 6.1.4.
If a character is identified as unreserved in RFC 2396 or appears in extraUnescaped, it is not
escaped. It performs the following steps when called:
Because percent-encoding is used to represent individual octets, a single code point may be expressed
as multiple consecutive escape sequences (one for each of its 8-bit UTF-8 code units).
19.2.6.6 Decode ( string, preserveEscapeSet )
The abstract operation Decode takes arguments string (a String) and
preserveEscapeSet (a String) and returns either a normal completion
containing a String or a throw
completion. It performs URI unescaping and decoding, preserving any escape sequences
that correspond to Basic Latin characters in preserveEscapeSet. It performs the following steps
when called:
1. Let len be the length of string.
2. Let R be the empty String.
3. Let k be 0.
4. Repeat, while k < len,
a. Let C be the code unit at index k
within string.
b. Let S be C.
c. If C is the code unit 0x0025 (PERCENT SIGN), then
i. If k + 3 > len, throw a
URIError exception.
ii. Let escape be the substring of
string from k to k + 3.
RFC 3629 prohibits the decoding of invalid UTF-8 octet sequences. For example, the invalid sequence
0xC0 0x80 must not decode into the code unit 0x0000. Implementations of the Decode algorithm are
required to throw a URIError when encountering such invalid sequences.
19.2.6.7 ParseHexOctet ( string, position )
The abstract operation ParseHexOctet takes arguments string (a String) and position
(a non-negative integer) and returns either a non-negative
integer or a non-empty List of
SyntaxError objects. It parses a sequence of two hexadecimal characters at the
specified position in string into an unsigned 8-bit integer. It performs the
following steps when called:
callbackfn should be a function that accepts two arguments. groupBy calls
callbackfn once for each element in items, in ascending order, and constructs a
new object. Each value returned by callbackfn is coerced to a property
key. For each such
property key, the result
object has a property whose key is that property key and whose value
is an array containing all the elements for which the callbackfn return value coerced to
that key.
callbackfn is called with two arguments: the value of the element and the index of the
element.
The return value of groupBy is an object that does not inherit from %Object.prototype%.
This function performs the following steps when called:
1. Let groups be ? GroupBy(items, callbackfn,
property).
The ordering of steps 1 and
2 is chosen to
ensure that any exception that would have been thrown by step 1 in
previous editions of this specification will continue to be thrown even if the this
value is undefined or null.
20.1.3.3 Object.prototype.isPrototypeOf ( V )
This method performs the following steps when called:
The ordering of steps 1 and
2 preserves the
behaviour specified by previous editions of this specification for the case where V is not
an object and the this value is undefined or
null.
20.1.3.4 Object.prototype.propertyIsEnumerable ( V )
This method performs the following steps when called:
This method does not consider objects in the prototype chain.
Note 2
The ordering of steps 1 and
2 is chosen
to ensure that any exception that would have been thrown by step 1 in
previous editions of this specification will continue to be thrown even if the this
value is undefined or null.
The optional parameters to this method are not used but are intended to correspond to the parameter
pattern used by ECMA-402 toLocaleString methods. Implementations that do not include ECMA-402
support must not use those parameter positions for other purposes.
Note 1
This method provides a generic toLocaleString implementation for objects that have no
locale-sensitive toString behaviour. Array, Number,
Date, and %TypedArray%
provide their own locale-sensitive toLocaleString methods.
Note 2
ECMA-402 intentionally does not provide an alternative to this default implementation.
20.1.3.6 Object.prototype.toString ( )
This method performs the following steps when called:
1. If the this value is
undefined, return "[object Undefined]".
2. If the this value is null,
return "[object Null]".
Historically, this method was occasionally used to access the String value of the [[Class]] internal slot that was used in previous editions of this specification
as a nominal type tag for various built-in objects. The above definition of toString
preserves compatibility for legacy code that uses toString as a test for those specific
kinds of built-in objects. It does not provide a reliable type testing mechanism for other kinds of
built-in or program defined objects. In addition, programs can use @@toStringTag in ways
that will invalidate the reliability of such legacy type tests.
20.1.3.7 Object.prototype.valueOf ( )
This method performs the following steps when called:
Object.prototype.__proto__ is an accessor property with attributes
{ [[Enumerable]]: false, [[Configurable]]: true }. The [[Get]]
and [[Set]] attributes are defined as follows:
20.1.3.8.1 get Object.prototype.__proto__
The value of the [[Get]] attribute is a built-in function that requires no
arguments. It performs the following steps when called:
is the initial value of the "Function" property of the global
object.
creates and initializes a new function object when called as a
function rather than as a constructor. Thus the function call
Function(…) is equivalent to the object creation expression new Function(…) with
the same arguments.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit
the specified Function behaviour must include a super call to the Function constructor to create and initialize a
subclass instance with the internal slots necessary for built-in function behaviour. All ECMAScript
syntactic forms for defining function objects create instances
of Function. There is no syntactic means to create instances of Function subclasses except for the
built-in GeneratorFunction, AsyncFunction, and AsyncGeneratorFunction subclasses.
20.2.1.1 Function ( ...parameterArgs, bodyArg )
The last argument (if any) specifies the body (executable code) of a function; any preceding arguments
specify formal parameters.
This function performs the following steps when called:
It is permissible but not necessary to have one argument for each formal parameter to be specified.
For example, all three of the following expressions produce the same result:
The abstract operation CreateDynamicFunction takes arguments constructor (a constructor), newTarget
(a constructor), kind
(normal, generator, async, or
async-generator), parameterArgs (a List of
ECMAScript language
values), and bodyArg (an ECMAScript
language value) and returns either a normal completion
containing an ECMAScript function object or a throw
completion. constructor is the constructor function
that is performing this action. newTarget is the constructor that
new was initially applied to. parameterArgs and bodyArg reflect the
argument values that were passed to constructor. It performs the following steps when called:
1. If newTarget is undefined, set
newTarget to constructor.
20. If body is a List
of errors, throw a SyntaxError exception.
21. NOTE: The parameters and body are parsed separately to ensure
that each is valid alone. For example, new Function("/*", "*/ ) {") does not evaluate
to a function.
22. NOTE: If this step is reached, sourceText must have
the syntax of exprSym (although the reverse implication does not hold). The purpose of
the next two steps is to enforce any Early Error rules which apply to exprSym directly.
33. NOTE: Functions whose kind is
async are not constructible and do not have a [[Construct]] internal method or a "prototype" property.
34. Return F.
Note
CreateDynamicFunction defines a "prototype" property on any function it creates
whose kind is not async to provide for the possibility that the
function will be used as a constructor.
has a "name" property whose value is the empty String.
Note
The Function prototype object is specified to be a function object to ensure
compatibility with ECMAScript code that was created prior to the ECMAScript 2015 specification.
The thisArg value is passed without modification as the this value.
This is a change from Edition 3, where an undefined or nullthisArg is replaced with the global object and ToObject is applied to all other
values and that result is passed as the this value. Even though the
thisArg is passed without modification, non-strict
functions still perform these transformations upon entry to the function.
Note 2
If func is either an arrow function or a bound function
exotic object, then the thisArg will be ignored by the function [[Call]] in step 6.
Function objects created using
Function.prototype.bind are exotic objects. They also do not
have a "prototype" property.
Note 2
If Target is either an arrow function or a bound function
exotic object, then the thisArg passed to this method will not be used
by subsequent calls to F.
The thisArg value is passed without modification as the this value.
This is a change from Edition 3, where an undefined or nullthisArg is replaced with the global object and ToObject is applied to all other
values and that result is passed as the this value. Even though the
thisArg is passed without modification, non-strict
functions still perform these transformations upon entry to the function.
Note 2
If func is either an arrow function or a bound function
exotic object, then the thisArg will be ignored by the function [[Call]] in step 4.
20.2.3.4 Function.prototype.constructor
The initial value of Function.prototype.constructor is %Function%.
20.2.3.5 Function.prototype.toString ( )
This method performs the following steps when called:
1. Let func be the this value.
2. If funcis an Object,
func has a [[SourceText]] internal slot, func.[[SourceText]] is a sequence of Unicode code points, and HostHasSourceTextAvailable(func)
is true, then
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.
Note
This is the default implementation of @@hasInstance that most functions inherit.
@@hasInstance is called by the instanceof operator to determine whether a
value is an instance of a specific constructor. An expression such as
v instanceof F
evaluates as
F[@@hasInstance](v)
A constructor function can control
which objects are recognized as its instances by instanceof by exposing a different
@@hasInstance method on the function.
This property is non-writable and non-configurable to prevent tampering that could be used to globally
expose the target function of a bound function.
The value of the "name" property of this method is
"[Symbol.hasInstance]".
20.2.4 Function Instances
Every Function instance is an ECMAScript function object and has the internal
slots listed in Table
30. Function objects created using the
Function.prototype.bind method (20.2.3.2) have the
internal slots listed in Table
31.
Function instances have the following properties:
20.2.4.1 length
The value of the "length" property is an integral Number
that indicates the typical number of arguments expected by the function. However, the language permits the
function to be invoked with some other number of arguments. The behaviour of a function when invoked on a
number of arguments other than the number specified by its "length" property depends on
the function. This property has the attributes { [[Writable]]:
false, [[Enumerable]]: false, [[Configurable]]: true }.
20.2.4.2 name
The value of the "name" property is a
String that is descriptive of the function. The name has no semantic significance but
is typically a variable or property name that is used to refer
to the function at its point of definition in ECMAScript source text. This
property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
true }.
Anonymous functions objects that do not have a contextual name associated with them by this specification
use the empty String as the value of the "name" property.
20.2.4.3 prototype
Function instances that can be used as a constructor have a
"prototype" property. Whenever such a Function instance is created another ordinary
object is also created and is the initial value of the function's
"prototype" property. Unless otherwise specified, the value of the
"prototype" property is used to initialize the [[Prototype]]
internal slot of the object created when that function is invoked as a constructor.
This property has the attributes { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]:
false }.
The host-defined abstract operation
HostHasSourceTextAvailable takes argument func (a function object) and
returns a Boolean. It allows host environments to prevent the
source text from being provided for func.
An implementation of HostHasSourceTextAvailable must conform to the following requirements:
It must be deterministic with respect to its parameters. Each time it is called with a specific
func as its argument, it must return the same result.
The default implementation of HostHasSourceTextAvailable is to return true.
is the initial value of the "Boolean" property of the global
object.
creates and initializes a new Boolean object when called as a constructor.
performs a type conversion when called as a function rather than as a constructor.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit
the specified Boolean behaviour must include a super call to the Boolean constructor to create and initialize
the subclass instance with a [[BooleanData]] internal slot.
20.3.1.1 Boolean ( value )
This function performs the following steps when called:
Boolean instances are ordinary objects that inherit
properties from the Boolean prototype
object. Boolean instances have a [[BooleanData]] internal slot.
The [[BooleanData]] internal slot is the Boolean value represented by this Boolean
object.
2. For each element e of the GlobalSymbolRegistry
List,
do
a. If SameValue(e.[[Key]], stringKey) is true, return
e.[[Symbol]].
3. Assert: GlobalSymbolRegistry does not
currently contain an entry for stringKey.
4. Let newSymbol be a new Symbol whose [[Description]] is stringKey.
5. Append the Record
{ [[Key]]: stringKey, [[Symbol]]:
newSymbol } to the GlobalSymbolRegistry List.
6. Return newSymbol.
The GlobalSymbolRegistry is an append-only List that
is globally available. It is shared by all realms. Prior to the evaluation of any
ECMAScript code, it is initialized as a new empty List.
Elements of the GlobalSymbolRegistry are Records
with the structure defined in Table 60.
The initial value of Symbol.prototype.constructor is %Symbol%.
20.4.3.2 get Symbol.prototype.description
Symbol.prototype.description is an accessor property whose set
accessor function is undefined. Its get accessor function performs the following steps
when called:
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
true }.
The value of the "name" property of this method is
"[Symbol.toPrimitive]".
20.4.3.6 Symbol.prototype [ @@toStringTag ]
The initial value of the @@toStringTag property is
the String value "Symbol".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
true }.
20.4.4 Properties of Symbol Instances
Symbol instances are ordinary objects that inherit
properties from the Symbol prototype
object. Symbol instances have a [[SymbolData]] internal slot.
The [[SymbolData]] internal slot is the Symbol value represented by this Symbol
object.
20.4.5 Abstract Operations for Symbols
20.4.5.1 KeyForSymbol ( sym )
The abstract operation KeyForSymbol takes argument sym (a Symbol) and returns a String or
undefined. If sym is in the GlobalSymbolRegistry (see 20.4.2.2) the String used to
register sym will be returned. It performs the following steps when called:
1. For each element e of the GlobalSymbolRegistry
List,
do
a. If SameValue(e.[[Symbol]], sym) is true, return
e.[[Key]].
2. Assert: GlobalSymbolRegistry does not
currently contain an entry for sym.
3. Return undefined.
20.5 Error Objects
Instances of Error objects are thrown as exceptions when runtime errors occur. The Error objects may also
serve as base objects for user-defined exception classes.
When an ECMAScript implementation detects a runtime error, it throws a new instance of one of the
NativeError objects defined in 20.5.5 or
a new instance of AggregateError object defined in 20.5.7.
Each of these objects has the structure described below, differing only in the name used as the constructor name instead of
NativeError, in the "name" property of the prototype object, in the implementation-defined"message" property of the prototype object, and in the presence of the %AggregateError%-specific
"errors" property.
is the initial value of the "Error" property of the global
object.
creates and initializes a new Error object when called as a function rather than as a constructor. Thus the function call
Error(…) is equivalent to the object creation expression new Error(…) with the
same arguments.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit
the specified Error behaviour must include a super call to the Error constructor to create and initialize
subclass instances with an [[ErrorData]] internal slot.
20.5.1.1 Error ( message [ , options ] )
This function performs the following steps when called:
1. If NewTarget is undefined, let
newTarget be the active function object;
else let newTarget be NewTarget.
6. If msg is undefined, set
msg to the empty String; otherwise set msg to ? ToString(msg).
7. If name is the empty String, return msg.
8. If msg is the empty String, return name.
9. Return the string-concatenation of
name, the code unit 0x003A (COLON), the code unit 0x0020 (SPACE), and msg.
20.5.4 Properties of Error Instances
Error instances are ordinary objects that inherit
properties from the Error prototype
object and have an [[ErrorData]] internal slot whose value is
undefined. The only specified uses of [[ErrorData]] is to
identify Error, AggregateError, and NativeError instances as Error objects within
Object.prototype.toString.
20.5.5 Native Error Types Used in This Standard
A new instance of one of the NativeError objects below or of the AggregateError object is thrown
when a runtime error is detected. All NativeError objects share the same structure, as described
in 20.5.6.
Indicates that one of the global URI handling functions was used in a way that is incompatible with its
definition.
20.5.6NativeError Object Structure
When an ECMAScript implementation detects a runtime error, it throws a new instance of one of the
NativeError objects defined in 20.5.5.
Each of these objects has the structure described below, differing only in the name used as the constructor name instead of
NativeError, in the "name" property of the prototype object, and in the
implementation-defined"message" property of the prototype object.
For each error object, references to NativeError in the definition should be replaced with the
appropriate error object name from 20.5.5.
creates and initializes a new NativeError object when called as a function rather than as a
constructor. A call of the object as
a function is equivalent to calling it as a constructor with the same arguments.
Thus the function call NativeError(…) is equivalent to the object creation
expression new NativeError(…) with the same arguments.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit
the specified NativeError behaviour must include a super call to the
NativeErrorconstructor to create and initialize
subclass instances with an [[ErrorData]] internal slot.
20.5.6.1.1NativeError ( message [ ,
options ] )
Each NativeError function performs the following steps when called:
1. If NewTarget is undefined, let
newTarget be the active function
object; else let newTarget be NewTarget.
The actual value of the string passed in step 2
is either "%EvalError.prototype%", "%RangeError.prototype%",
"%ReferenceError.prototype%", "%SyntaxError.prototype%",
"%TypeError.prototype%", or "%URIError.prototype%" corresponding
to which NativeErrorconstructor is being defined.
20.5.6.2 Properties of the NativeError Constructors
The initial value of the "constructor" property of the prototype for a given
NativeErrorconstructor is the constructor itself.
20.5.6.3.2NativeError.prototype.message
The initial value of the "message" property of the prototype for a given
NativeErrorconstructor is the empty String.
20.5.6.3.3NativeError.prototype.name
The initial value of the "name" property of the prototype for a given
NativeErrorconstructor is the String value
consisting of the name of the constructor (the name used instead
of NativeError).
20.5.6.4 Properties of NativeError Instances
NativeError instances are ordinary objects that inherit
properties from their NativeError prototype object and have an [[ErrorData]] internal slot whose value is undefined. The only
specified use of [[ErrorData]] is by Object.prototype.toString
(20.1.3.6) to
identify Error, AggregateError, or NativeError instances.
is the initial value of the "AggregateError" property of the global
object.
creates and initializes a new AggregateError object when called as a function rather than as a
constructor. Thus the function call
AggregateError(…) is equivalent to the object creation expression
new AggregateError(…) with the same arguments.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit
the specified AggregateError behaviour must include a super call to the AggregateError
constructor to create and initialize
subclass instances with an [[ErrorData]] internal slot.
The initial value of AggregateError.prototype.constructor is %AggregateError%.
20.5.7.3.2 AggregateError.prototype.message
The initial value of AggregateError.prototype.message is the empty String.
20.5.7.3.3 AggregateError.prototype.name
The initial value of AggregateError.prototype.name is "AggregateError".
20.5.7.4 Properties of AggregateError Instances
AggregateError instances are ordinary objects that inherit
properties from their AggregateError
prototype object and have an [[ErrorData]] internal slot whose
value is undefined. The only specified use of [[ErrorData]] is
by Object.prototype.toString (20.1.3.6)
to
identify Error, AggregateError, or NativeError instances.
20.5.8 Abstract Operations for Error Objects
20.5.8.1 InstallErrorCause ( O, options )
The abstract operation InstallErrorCause takes arguments O (an Object) and options
(an ECMAScript language
value) and returns either a normal
completion containingunused or a throw
completion. It is used to create a "cause" property on O
when a "cause" property is present on options. It performs the following
steps when called:
is the initial value of the "Number" property of the global
object.
creates and initializes a new Number object when called as a constructor.
performs a type conversion when called as a function rather than as a constructor.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit
the specified Number behaviour must include a super call to the Number constructor to create and initialize
the subclass instance with a [[NumberData]] internal slot.
21.1.1.1 Number ( value )
This function performs the following steps when called:
The value of Number.EPSILON is the Number value for the magnitude of
the difference between 1 and the smallest value greater than 1 that is representable as a Number value,
which is approximately 2.2204460492503130808472633361816 × 10**-16.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.
21.1.2.2 Number.isFinite ( number )
This function performs the following steps when called:
This function differs from the global isNaN function (19.2.3) in
that it does not convert its argument to a Number before determining whether it is
NaN.
Due to rounding behaviour necessitated by precision limitations of IEEE
754-2019, the Number value for every
integer greater than
Number.MAX_SAFE_INTEGER is shared with at least one other integer. Such
large-magnitude integers are therefore not safe, and are not guaranteed to
be exactly representable as Number values or even to be distinguishable from each other. For example,
both 9007199254740992 and 9007199254740993 evaluate to the Number value
9007199254740992𝔽.
The value of Number.MAX_SAFE_INTEGER is 9007199254740991𝔽
(𝔽(2**53 - 1)).
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.
21.1.2.7 Number.MAX_VALUE
The value of Number.MAX_VALUE is the largest positive finite value of the
Number
type, which is approximately 1.7976931348623157 × 10**308.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.
21.1.2.8 Number.MIN_SAFE_INTEGER
Note
Due to rounding behaviour necessitated by precision limitations of IEEE
754-2019, the Number value for every
integer less than
Number.MIN_SAFE_INTEGER is shared with at least one other integer. Such
large-magnitude integers are therefore not safe, and are not guaranteed to
be exactly representable as Number values or even to be distinguishable from each other. For example,
both -9007199254740992 and -9007199254740993 evaluate to the Number value
-9007199254740992𝔽.
The value of Number.MIN_SAFE_INTEGER is -9007199254740991𝔽
(𝔽(-(2**53 - 1))).
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.
21.1.2.9 Number.MIN_VALUE
The value of Number.MIN_VALUE is the smallest positive value of the Number
type, which is approximately 5 × 10**-324.
In the IEEE 754-2019 double precision
binary representation, the smallest possible value is a denormalized number. If an implementation does not
support denormalized values, the value of Number.MIN_VALUE must be the smallest non-zero
positive value that can actually be represented by the implementation.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.
21.1.2.10 Number.NaN
The value of Number.NaN is NaN.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.
21.1.2.11 Number.NEGATIVE_INFINITY
The value of Number.NEGATIVE_INFINITY is -∞𝔽.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.
21.1.2.12 Number.parseFloat ( string )
The initial value of the "parseFloat" property is %parseFloat%.
21.1.2.13 Number.parseInt ( string, radix )
The initial value of the "parseInt" property is %parseInt%.
21.1.2.14 Number.POSITIVE_INFINITY
The value of Number.POSITIVE_INFINITY is +∞𝔽.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.
Unless explicitly stated otherwise, the methods of the Number prototype object defined below are not
generic and the this value passed to them must be either a Number value or an object that
has a [[NumberData]] internal slot that has been initialized to a Number value.
The phrase “this Number value” within the specification of a method refers to the result returned by
calling the abstract operation ThisNumberValue with the
this value of the method invocation passed as the argument.
21.1.3.1 Number.prototype.constructor
The initial value of Number.prototype.constructor is %Number%.
This method returns a String containing this Number value represented in decimal exponential notation
with one digit before the significand's decimal point and fractionDigits digits after the
significand's decimal point. If fractionDigits is undefined, it includes as
many significand digits as necessary to uniquely specify the Number (just like in ToString
except that in this case the
Number is always output in exponential notation).
a. Let m be the String value consisting of
f + 1 occurrences of the code unit 0x0030 (DIGIT ZERO).
b. Let e be 0.
10. Else,
a. If fractionDigits is not
undefined, then
i. Let e and n be integers such that 10**f ≤ n < 10**(f + 1)
and for which n × 10**(e -
f) - x is as close to zero as
possible. If there are two such sets of e and n, pick the e
and n for which n × 10**(e - f) is larger.
b. Else,
i.
Let e, n, and ff be integers such
that ff ≥ 0, 10**ff ≤
n < 10**(ff + 1), 𝔽(n × 10**(e - ff)) is 𝔽(x), and
ff is as small as possible. Note that the decimal representation of n
has ff + 1 digits, n is not divisible by 10, and the least significant
digit of n is not necessarily uniquely determined by these criteria.
ii. Set f to ff.
c. Let m be the String value consisting of the digits
of the decimal representation of n (in order, with no leading zeroes).
For implementations that provide more accurate conversions than required by the rules above, it is
recommended that the following alternative version of step 10.b.i
be used as a guideline:
i. Let e, n, and f be integers such that f ≥
0, 10**f ≤ n < 10**(f + 1),
𝔽(n × 10**(e - f)) is 𝔽(x), and
f is as small as possible. If there are multiple possibilities for n, choose
the value of n for which 𝔽(n × 10**(e - f)) is closest in value to 𝔽(x). If there
are two such possible values of n, choose the one that is even.
This method returns a String containing this Number value represented in decimal fixed-point notation
with fractionDigits digits after the decimal point. If fractionDigits is
undefined, 0 is assumed.
a. Let n be an integer for which
n / 10**f - x is as
close to zero as possible. If there are two such n, pick the larger n.
b. If n = 0, let m be
"0". Otherwise, let m be the String value consisting of the digits
of the decimal representation of n (in order, with no leading zeroes).
c. If f ≠ 0, then
i. Let k be the length of m.
ii. If k ≤ f, then
1. Let z be the String value consisting of
f + 1 - k occurrences of the code unit 0x0030 (DIGIT ZERO).
The output of toFixed may be more precise than toString for some values
because toString only prints enough significant digits to distinguish the number from adjacent Number
values. For example,
(1000000000000000128).toString() returns "1000000000000000100",
while (1000000000000000128).toFixed(0) returns "1000000000000000128".
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement this
method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the
ECMA-402 API the following specification of this method is used:
This method produces a String value that represents this Number value formatted according to the
conventions of the host environment's current
locale. This method is implementation-defined, and
it is permissible, but not encouraged, for it to return the same thing as toString.
The meanings of the optional parameters to this method are defined in the ECMA-402 specification;
implementations that do not include ECMA-402 support must not use those parameter positions for anything
else.
This method returns a String containing this Number value represented either in decimal exponential
notation with one digit before the significand's decimal point and precision - 1 digits after the significand's decimal point or in
decimal fixed notation with precision significant digits. If precision is
undefined, it calls ToString instead.
a. Let m be the String value consisting of
p occurrences of the code unit 0x0030 (DIGIT ZERO).
b. Let e be 0.
10. Else,
a. Let e and n be integers such that 10**(p - 1) ≤
n < 10**p and for which
n × 10**(e - p +
1) - x is as close to zero as possible. If
there are two such sets of e and n, pick the e and n
for which n × 10**(e - p +
1) is larger.
b. Let m be the String value consisting of the digits
of the decimal representation of n (in order, with no leading zeroes).
a. Set m to the string-concatenation
of the first e + 1 code units of m, the code unit 0x002E (FULL STOP), and
the remaining p - (e + 1) code units of m.
13. Else,
a. Set m to the string-concatenation
of the code unit 0x0030 (DIGIT ZERO), the code unit 0x002E (FULL STOP), -(e + 1)
occurrences of the code unit 0x0030 (DIGIT ZERO), and the String m.
The optional radix should be an integral Number value in the
inclusive interval from
2𝔽 to 36𝔽. If radix is
undefined then 10𝔽 is used as the value of
radix.
This method performs the following steps when called:
This method is not generic; it throws a TypeError exception if its
this value is not
a Number or a Number object. Therefore, it cannot be transferred to other kinds of
objects for use as a method.
Number instances are ordinary objects that inherit
properties from the Number prototype
object. Number instances also have a [[NumberData]] internal
slot. The [[NumberData]] internal slot is the Number value represented by this
Number object.
is the initial value of the "BigInt" property of the global
object.
performs a type conversion when called as a function rather than as a constructor.
is not intended to be used with the new operator or to be subclassed. It may be used as the
value of an extends clause of a class definition but a super call to the BigInt
constructor will cause an exception.
21.2.1.1 BigInt ( value )
This function performs the following steps when called:
1. If NewTarget is not undefined, throw a
TypeError exception.
The abstract operation NumberToBigInt takes argument number (a Number) and returns either a
normal completion
containing a BigInt or a throw
completion. It performs the following steps when called:
1. If IsIntegralNumber(number)
is false, throw a RangeError exception.
The phrase “this BigInt value” within the specification of a method refers to the result returned by
calling the abstract operation ThisBigIntValue with the
this value of the method invocation passed as the argument.
21.2.3.1 BigInt.prototype.constructor
The initial value of BigInt.prototype.constructor is %BigInt%.
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement this
method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the
ECMA-402 API the following specification of this method is used:
This method produces a String value that represents this BigInt value formatted according to the
conventions of the host environment's current
locale. This method is implementation-defined, and
it is permissible, but not encouraged, for it to return the same thing as toString.
The meanings of the optional parameters to this method are defined in the ECMA-402 specification;
implementations that do not include ECMA-402 support must not use those parameter positions for anything
else.
21.2.3.3 BigInt.prototype.toString ( [ radix ] )
Note
The optional radix should be an integral Number value in the
inclusive interval from
2𝔽 to 36𝔽. If radix is
undefined then 10𝔽 is used as the value of
radix.
This method performs the following steps when called:
This method is not generic; it throws a TypeError exception if its
this value is not
a BigInt or a BigInt object. Therefore, it cannot be transferred to other kinds of
objects for use as a method.
The initial value of the @@toStringTag property is
the String value "BigInt".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
true }.
21.2.4 Properties of BigInt Instances
BigInt instances are ordinary objects that inherit
properties from the BigInt prototype
object. BigInt instances also have a [[BigIntData]] internal
slot. The [[BigIntData]] internal slot is the BigInt value represented by this
BigInt object.
21.3 The Math Object
The Math object:
is %Math%.
is the initial value of the "Math" property of the global object.
does not have a [[Construct]] internal method; it cannot be used as a constructor with the new
operator.
does not have a [[Call]] internal method; it cannot be invoked as a function.
Note
In this specification, the phrase “the Number value forx”
has a technical meaning defined in 6.1.6.1.
21.3.1 Value Properties of the Math Object
21.3.1.1 Math.E
The Number value fore, the
base of the natural logarithms, which is approximately 2.7182818284590452354.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.
21.3.1.2 Math.LN10
The Number value for the natural
logarithm of 10, which is approximately 2.302585092994046.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.
21.3.1.3 Math.LN2
The Number value for the natural
logarithm of 2, which is approximately 0.6931471805599453.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.
21.3.1.4 Math.LOG10E
The Number value for the base-10
logarithm of e, the base of the natural logarithms; this value is approximately 0.4342944819032518.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.
Note
The value of Math.LOG10E is approximately the reciprocal of the value of
Math.LN10.
21.3.1.5 Math.LOG2E
The Number value for the base-2
logarithm of e, the base of the natural logarithms; this value is approximately 1.4426950408889634.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.
Note
The value of Math.LOG2E is approximately the reciprocal of the value of
Math.LN2.
21.3.1.6 Math.PI
The Number value for π, the ratio of
the circumference of a circle to its diameter, which is approximately 3.1415926535897932.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.
21.3.1.7 Math.SQRT1_2
The Number value for the square root
of ½, which is approximately 0.7071067811865476.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.
Note
The value of Math.SQRT1_2 is approximately the reciprocal of the value of
Math.SQRT2.
21.3.1.8 Math.SQRT2
The Number value for the square root
of 2, which is approximately 1.4142135623730951.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.
21.3.1.9 Math [ @@toStringTag ]
The initial value of the @@toStringTag property is
the String value "Math".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
true }.
21.3.2 Function Properties of the Math Object
Note
The behaviour of the functions acos, acosh, asin,
asinh, atan, atanh, atan2, cbrt,
cos, cosh, exp, expm1, hypot,
log, log1p, log2, log10, pow,
random, sin, sinh, sqrt, tan, and
tanh is not precisely specified here except to require specific results for certain
argument values that represent boundary cases of interest. For other argument values, these functions
are intended to compute approximations to the results of familiar mathematical functions, but some
latitude is allowed in the choice of approximation algorithms. The general intent is that an implementer
should be able to use the same mathematical library for ECMAScript on a given hardware platform that is
available to C programmers on that platform.
Although the choice of algorithms is left to the implementation, it is recommended (but not specified
by this standard) that implementations use the approximation algorithms for IEEE
754-2019 arithmetic contained in fdlibm, the freely distributable
mathematical library from Sun Microsystems (http://www.netlib.org/fdlibm).
21.3.2.1 Math.abs ( x )
This function returns the absolute value of x; the result has the same magnitude as
x but has positive sign.
6. Return an implementation-approximated
Number value representing the result of the inverse hyperbolic tangent of ℝ(n).
21.3.2.8 Math.atan2 ( y, x )
This function returns the inverse tangent of the quotient y /
x of the arguments y and x, where the signs of y
and x are used to determine the quadrant of the result. Note that it is intentional and
traditional for the two-argument inverse tangent function that the argument named y be first
and the argument named x be second. The result is expressed in radians and is in the inclusive interval from -π to
+π.
5. Return an implementation-approximated
Number value representing the result of the exponential function of ℝ(n).
21.3.2.15 Math.expm1 ( x )
This function returns the result of subtracting 1 from the exponential function of x
(e raised to the power of x, where e is the base of the natural
logarithms). The result is computed in a way that is accurate even when the value of x is close
to 0.
2. If n is one of NaN,
+0𝔽, -0𝔽, or
+∞𝔽, return n.
3. If n is -∞𝔽, return
-1𝔽.
4. Return an implementation-approximated
Number value representing the result of subtracting 1 from the exponential function of ℝ(n).
21.3.2.16 Math.floor ( x )
This function returns the greatest (closest to +∞) integral Number value that is not
greater than x. If x is already an integral Number,
the result is x.
Implementations should take care to avoid the loss of precision from overflows and underflows that
are prone to occur in naive implementations when this function is called with two or more arguments.
21.3.2.19 Math.imul ( x, y )
This function performs the following steps when called:
This function returns a Number value with positive sign, greater than or equal to
+0𝔽 but strictly less than 1𝔽, chosen
randomly or pseudo randomly with approximately uniform distribution over that range, using an implementation-defined
algorithm or strategy.
Each Math.random function created for distinct realms must produce a distinct sequence of
values from successive calls.
21.3.2.28 Math.round ( x )
This function returns the Number value that is closest to x and is integral. If two integral
Numbers are equally close to x, then the result is the Number value that is
closer to +∞. If x is already integral, the result is x.
5. Return the integral Number closest to
n, preferring the Number closer to +∞ in the case of a tie.
Note 1
Math.round(3.5) returns 4, but Math.round(-3.5) returns -3.
Note 2
The value of Math.round(x) is not always the same as the value of
Math.floor(x + 0.5). When x is -0𝔽 or
x is less than +0𝔽 but greater than or equal to
-0.5𝔽, Math.round(x) returns
-0𝔽, but Math.floor(x + 0.5) returns
+0𝔽. Math.round(x) may also differ from the value of
Math.floor(x + 0.5)because of internal rounding when computing x + 0.5.
21.3.2.29 Math.sign ( x )
This function returns the sign of x, indicating whether x is positive, negative, or
zero.
2. If n is not finite or n is either
+0𝔽 or -0𝔽, return n.
3. If n < 1𝔽 and
n > +0𝔽, return +0𝔽.
4. If n < -0𝔽 and
n > -1𝔽, return -0𝔽.
5. Return the integral Number nearest
n in the direction of +0𝔽.
21.4 Date Objects
21.4.1 Overview of Date Objects and Definitions of Abstract Operations
The following abstract
operations operate on time values (defined
in 21.4.1.1). Note that,
in every case, if any argument to one of these functions is NaN, the result will be
NaN.
21.4.1.1 Time Values and Time Range
Time measurement in ECMAScript is analogous to time measurement in POSIX, in particular sharing
definition in terms of the proleptic Gregorian calendar, an epoch of
midnight at the beginning of 1 January 1970 UTC, and an accounting of every day as comprising exactly
86,400 seconds (each of which is 1000 milliseconds long).
An ECMAScript time value is a
Number, either a finiteintegral Number
representing an instant in time to millisecond precision or NaN representing no
specific instant. A time value that is a multiple of 24 × 60 × 60 × 1000 =
86,400,000 (i.e., is 86,400,000 × d for some integerd)
represents the instant at the start of the UTC day that follows the epoch by d whole
UTC days (preceding the epoch for negative d). Every
other finite time value t is defined
relative to the greatest preceding time value s that is such a multiple, and represents the
instant that occurs within the same UTC day as s but follows it by (t -
s) milliseconds.
Time values do not account for UTC leap seconds—there are no time values representing instants within
positive leap seconds, and there are time values representing instants removed from the UTC timeline by
negative leap seconds. However, the definition of time values nonetheless yields piecewise alignment with
UTC, with discontinuities only at leap second boundaries and zero difference outside of leap seconds.
A Number can exactly represent all integers from -9,007,199,254,740,992 to
9,007,199,254,740,992 (21.1.2.8 and 21.1.2.6). A time
value supports a slightly smaller range of -8,640,000,000,000,000 to 8,640,000,000,000,000 milliseconds.
This yields a supported time value range of exactly -100,000,000 days to 100,000,000 days relative to
midnight at the beginning of 1 January 1970 UTC.
The exact moment of midnight at the beginning of 1 January 1970 UTC is represented by the time value
+0𝔽.
Note
In the proleptic Gregorian calendar, leap years are precisely those which are both divisible by 4 and
either divisible by 400 or not divisible by 100.
The 400 year cycle of the proleptic Gregorian calendar contains 97 leap years. This yields an average
of 365.2425 days per year, which is 31,556,952,000 milliseconds. Therefore, the maximum range a Number
could represent exactly with millisecond precision is approximately -285,426 to 285,426 years relative
to 1970. The smaller range supported by a time value as specified in this section is approximately
-273,790 to 273,790 years relative to 1970.
21.4.1.2 Time-related Constants
These constants are referenced by algorithms in the following sections.
The abstract operation Day takes argument t (a finitetime value) and
returns an integral Number. It returns the
day number of the day in which t falls. It performs the following steps when called:
The abstract operation TimeWithinDay takes argument t (a finitetime value) and
returns an integral Number in the interval
from +0𝔽 (inclusive) to msPerDay (exclusive). It returns the
number of milliseconds since the start of the day in which t falls. It performs the following
steps when called:
The abstract operation DaysInYear takes argument y (an integral Number)
and returns 365𝔽 or 366𝔽. It returns the
number of days in year y. Leap years have 366 days; all other years have 365. It performs the
following steps when called:
The abstract operation DayFromYear takes argument y (an integral Number)
and returns an integral Number. It returns the
day number of the first day of year y. It performs the following steps when called:
2. NOTE: In
the following steps, numYears1, numYears4, numYears100, and
numYears400 represent the number of years divisible by 1, 4, 100, and 400, respectively,
that occur between the epoch and the start of year
y. The number is negative if y is before the epoch.
The abstract operation TimeFromYear takes argument y (an integral Number)
and returns a time value. It
returns the time value of the
start of year y. It performs the following steps when called:
The abstract operation YearFromTime takes argument t (a finitetime value) and
returns an integral Number. It returns the
year in which t falls. It performs the following steps when called:
The abstract operation InLeapYear takes argument t (a finitetime value) and
returns +0𝔽 or 1𝔽. It returns
1𝔽 if t is within a leap year and
+0𝔽 otherwise. It performs the following steps when called:
The abstract operation MonthFromTime takes argument t (a finitetime value) and
returns an integral Number in the inclusive interval from
+0𝔽 to 11𝔽. It returns a Number identifying
the month in which t falls. A month value of +0𝔽 specifies
January; 1𝔽 specifies February; 2𝔽 specifies
March; 3𝔽 specifies April; 4𝔽 specifies May;
5𝔽 specifies June; 6𝔽 specifies July;
7𝔽 specifies August; 8𝔽 specifies September;
9𝔽 specifies October; 10𝔽 specifies
November; and 11𝔽 specifies December. Note that MonthFromTime(+0𝔽) =
+0𝔽, corresponding to Thursday, 1 January 1970. It performs the
following steps when called:
The abstract operation DateFromTime takes argument t (a finitetime value) and
returns an integral Number in the inclusive interval from
1𝔽 to 31𝔽. It returns the day of the month
in which t falls. It performs the following steps when called:
The abstract operation WeekDay takes argument t (a finitetime value) and
returns an integral Number in the inclusive interval from
+0𝔽 to 6𝔽. It returns a Number identifying
the day of the week in which t falls. A weekday value of +0𝔽
specifies Sunday; 1𝔽 specifies Monday; 2𝔽
specifies Tuesday; 3𝔽 specifies Wednesday;
4𝔽 specifies Thursday; 5𝔽 specifies Friday;
and 6𝔽 specifies Saturday. Note that WeekDay(+0𝔽) =
4𝔽, corresponding to Thursday, 1 January 1970. It performs the
following steps when called:
The abstract operation HourFromTime takes argument t (a finitetime value) and
returns an integral Number in the inclusive interval from
+0𝔽 to 23𝔽. It returns the hour of the day
in which t falls. It performs the following steps when called:
The abstract operation MinFromTime takes argument t (a finitetime value) and
returns an integral Number in the inclusive interval from
+0𝔽 to 59𝔽. It returns the minute of the
hour in which t falls. It performs the following steps when called:
The abstract operation SecFromTime takes argument t (a finitetime value) and
returns an integral Number in the inclusive interval from
+0𝔽 to 59𝔽. It returns the second of the
minute in which t falls. It performs the following steps when called:
The abstract operation msFromTime takes argument t (a finitetime value) and
returns an integral Number in the inclusive interval from
+0𝔽 to 999𝔽. It returns the millisecond of
the second in which t falls. It performs the following steps when called:
Time zones in ECMAScript are represented by time zone
identifiers, which are Strings composed entirely of code units in the inclusive interval from 0x0000
to 0x007F.
Time zones supported by an ECMAScript implementation may be available named time zones, represented by the [[Identifier]] field of the Time Zone
Identifier Records returned by AvailableNamedTimeZoneIdentifiers,
or offset time zones, represented by Strings for
which IsTimeZoneOffsetString
returns true.
A primary time zone identifier is the
preferred identifier for an available named time zone.
A non-primary time zone identifier
is an identifier for an available named time zone that is not a primary time zone identifier.
An available named time zone
identifier is either a primary time zone identifier or a non-primary time zone identifier.
Each available named time zone identifier is associated with exactly one available named time zone.
Each available named time zone is associated with exactly one primary time zone identifier and zero or
more non-primary time zone identifiers.
ECMAScript implementations must support an available named time zone with the identifier
"UTC", which must be the primary time zone identifier for the UTC time zone.
In addition, implementations may support any number of other available named time zones.
Implementations that follow the requirements for time zones as described in the ECMA-402
Internationalization API specification are called time zone aware.
Time zone aware implementations must support available named time zones corresponding to the Zone and Link
names of the IANA Time Zone Database, and only such names.
In time zone aware implementations, a primary time zone identifier is a Zone name, and a non-primary time
zone identifier is a Link name, respectively, in the IANA Time Zone Database except as specifically
overridden by AvailableNamedTimeZoneIdentifiers
as specified in the ECMA-402 specification.
Implementations that do not support the entire IANA Time Zone Database are still recommended to use IANA
Time Zone Database names as identifiers to represent time zones.
When the input represents a local time occurring more than once because of a negative time zone transition
(e.g. when daylight saving time ends or the time zone offset is decreased due to a time zone rule change),
the returned List will
have more than one element and will be sorted by ascending numerical value.
When the input represents a local time skipped because of a positive time zone transition (e.g. when
daylight saving time begins or the time zone offset is increased due to a time zone rule change), the
returned List will
be empty.
Otherwise, the returned List will
have one element.
The default implementation of GetNamedTimeZoneEpochNanoseconds, to be used for ECMAScript implementations
that do not include local political rules for any time zones, performs the following steps when called:
1:30 AM on 5 November 2017 in America/New_York is repeated twice, so
GetNamedTimeZoneEpochNanoseconds("America/New_York", 2017, 11, 5, 1, 30, 0, 0, 0,
0) would return a List of
length 2 in which the first element represents 05:30 UTC (corresponding with 01:30 US Eastern Daylight
Time at UTC offset -04:00) and the second element represents 06:30 UTC (corresponding with 01:30 US
Eastern Standard Time at UTC offset -05:00).
2:30 AM on 12 March 2017 in America/New_York does not exist, so
GetNamedTimeZoneEpochNanoseconds("America/New_York", 2017, 3, 12, 2, 30, 0, 0, 0,
0) would return an empty List.
The implementation-defined
abstract operation GetNamedTimeZoneOffsetNanoseconds takes arguments timeZoneIdentifier (a
String) and epochNanoseconds (a BigInt) and returns an integer.
The returned integer represents the offset from UTC of
the named time zone identified by timeZoneIdentifier, at the instant corresponding with
epochNanoseconds relative to the epoch, both in nanoseconds.
The default implementation of GetNamedTimeZoneOffsetNanoseconds, to be used for ECMAScript
implementations that do not include local political rules for any time zones, performs the following steps
when called:
Time zone aware
implementations, including all implementations that implement the ECMA-402 Internationalization API, must
implement the AvailableNamedTimeZoneIdentifiers abstract operation as specified in the ECMA-402
specification.
For implementations that are not time zone aware,
AvailableNamedTimeZoneIdentifiers performs the following steps when called:
1. If the implementation does not include local political rules for
any time zones, then
The implementation-defined
abstract operation SystemTimeZoneIdentifier takes no arguments and returns a String.
It returns a String representing the host environment's current time
zone, which is either a String representing a UTC offset for which IsTimeZoneOffsetString
returns true, or a primary time zone
identifier.
It performs the following steps when called:
1. If the implementation only supports the UTC time zone, return
"UTC".
To ensure the level of functionality that implementations commonly provide in the methods of the Date
object, it is recommended that SystemTimeZoneIdentifier return an IANA time zone name corresponding to
the host environment's time zone
setting, if such a thing exists.
GetNamedTimeZoneEpochNanoseconds
and GetNamedTimeZoneOffsetNanoseconds
must reflect the local political rules for standard time and daylight saving time in that time zone,
if such rules exist.
For example, if the host environment is a browser
on a system where the user has chosen US Eastern Time as their time zone, SystemTimeZoneIdentifier
returns "America/New_York".
21.4.1.25 LocalTime ( t )
The abstract operation LocalTime takes argument t (a finitetime value) and
returns an integral Number.
It converts t from UTC to local time.
The local political rules for standard time and daylight saving time in effect at t should be
used to determine the result in the way specified in this section.
It performs the following steps when called:
Two different input time valuestUTC are converted to the same local time
tlocal at a negative time zone transition when there are
repeated times (e.g. the daylight saving time ends or the time zone adjustment is decreased.).
LocalTime(UTC(tlocal))
is not necessarily always equal to tlocal.
Correspondingly, UTC(LocalTime(tUTC))
is not necessarily always equal to tUTC.
21.4.1.26 UTC ( t )
The abstract operation UTC takes argument t (a Number) and returns a time value.
It converts t from local time to a UTC time
value.
The local political rules for standard time and daylight saving time in effect at t should be
used to determine the result in the way specified in this section.
It performs the following steps when called:
b. NOTE: The following steps ensure that when t
represents local time repeating multiple times at a negative time zone transition (e.g. when the
daylight saving time ends or the time zone offset is decreased due to a time zone rule change) or
skipped local time at a positive time zone transition (e.g. when the daylight saving time starts
or the time zone offset is increased due to a time zone rule change), t is interpreted
using the time zone offset before the transition.
c. If possibleInstants is not empty, then
i. Let disambiguatedInstant be
possibleInstants[0].
d. Else,
i. NOTE: t represents a local time skipped at a
positive time zone transition (e.g. due to daylight saving time starting or a time zone rule
change increasing the UTC offset).
Input t is nominally a time value but may
be any Number value.
The algorithm must not limit t to the time
value range, so that inputs corresponding with a boundary of the time value range
can be supported regardless of local UTC offset.
For example, the maximum time value is 8.64
× 10**15, corresponding with
"+275760-09-13T00:00:00Z".
In an environment where the local time zone offset is ahead of UTC by 1 hour at that instant, it is
represented by the larger input of 8.64 × 10**15 + 3.6 ×
10**6, corresponding with
"+275760-09-13T01:00:00+01:00".
1:30 AM on 5 November 2017 in America/New_York is repeated twice (fall backward), but it must be
interpreted as 1:30 AM UTC-04 instead of 1:30 AM UTC-05.
In UTC(TimeClip(MakeDate(MakeDay(2017, 10,
5), MakeTime(1, 30, 0, 0)))), the
value of offsetMs is -4 × msPerHour.
2:30 AM on 12 March 2017 in America/New_York does not exist, but it must be interpreted as 2:30 AM
UTC-05 (equivalent to 3:30 AM UTC-04).
In UTC(TimeClip(MakeDate(MakeDay(2017, 2,
12), MakeTime(2, 30, 0, 0)))), the
value of offsetMs is -5 × msPerHour.
Note 2
UTC(LocalTime(tUTC))
is not necessarily always equal to tUTC.
Correspondingly, LocalTime(UTC(tlocal))
is not necessarily always equal to tlocal.
21.4.1.27 MakeTime ( hour, min, sec,
ms )
The abstract operation MakeTime takes arguments hour (a Number), min (a Number),
sec (a Number), and ms (a Number) and returns a Number. It calculates a number of
milliseconds. It performs the following steps when called:
1. If hour is not finite, min
is not finite, sec is not finite,
or ms is not finite, return NaN.
The arithmetic in MakeTime is floating-point arithmetic, which is not associative, so the operations
must be performed in the correct order.
21.4.1.28 MakeDay ( year, month, date )
The abstract operation MakeDay takes arguments year (a Number), month (a Number),
and date (a Number) and returns a Number. It calculates a number of days. It performs the
following steps when called:
1. If year is not finite,
month is not finite, or date is not
finite, return NaN.
The abstract operation MakeDate takes arguments day (a Number) and time (a Number)
and returns a Number. It calculates a number of milliseconds. It performs the following steps when called:
1. If day is not finite or
time is not finite, return NaN.
The abstract operation MakeFullYear takes argument year (a Number) and returns an integral
Number or NaN. It returns the full year associated with the
integer part of year,
interpreting any value in the inclusive interval from 0 to 99
as a count of years since the start of 1900. For alignment with the proleptic Gregorian calendar, "full
year" is defined as the signed count of complete years since the start of year 0 (1 B.C.). It performs the
following steps when called:
The abstract operation TimeClip takes argument time (a Number) and returns a Number. It
calculates a number of milliseconds. It performs the following steps when called:
ECMAScript defines a string interchange format for date-times based upon a simplification of the ISO 8601
calendar date extended format. The format is as follows: YYYY-MM-DDTHH:mm:ss.sssZ
Where the elements are as follows:
YYYY
is the year in the proleptic Gregorian calendar as four decimal digits from 0000 to 9999, or as an
expanded year of
"+" or "-" followed by six decimal digits.
-
"-" (hyphen) appears literally twice in the string.
MM
is the month of the year as two decimal digits from 01 (January) to 12 (December).
DD
is the day of the month as two decimal digits from 01 to 31.
T
"T" appears literally in the string, to indicate the beginning of the time
element.
HH
is the number of complete hours that have passed since midnight as two decimal digits from 00 to
24.
:
":" (colon) appears literally twice in the string.
mm
is the number of complete minutes since the start of the hour as two decimal digits from 00 to 59.
ss
is the number of complete seconds since the start of the minute as two decimal digits from 00 to
59.
.
"." (dot) appears literally in the string.
sss
is the number of complete milliseconds since the start of the second as three decimal digits.
Z
is the UTC offset representation specified as "Z" (for UTC with no offset) or
as either "+" or "-" followed by a time expression
HH:mm (a subset of the time zone
offset string format for indicating local time ahead of or behind UTC,
respectively)
This format includes date-only forms:
YYYY
YYYY-MM
YYYY-MM-DD
It also includes “date-time” forms that consist of one of the above date-only forms immediately followed
by one of the following time forms with an optional UTC offset representation appended:
THH:mm
THH:mm:ss
THH:mm:ss.sss
A string containing out-of-bounds or nonconforming elements is not a valid instance of this format.
Note 1
As every day both starts and ends with midnight, the two notations 00:00 and
24:00 are available to distinguish the two midnights that can be associated with one
date. This means that the following two notations refer to exactly the same point in time:
1995-02-04T24:00 and 1995-02-05T00:00. This interpretation of the latter
form as "end of a calendar day" is consistent with ISO 8601, even though that specification reserves
it for describing time intervals and does not permit it within representations of single points in
time.
Note 2
There exists no international standard that specifies abbreviations for civil time zones like CET,
EST, etc. and sometimes the same abbreviation is even used for two very different time zones. For this
reason, both ISO 8601 and this format specify numeric representations of time zone offsets.
21.4.1.32.1 Expanded Years
Covering the full time value range
of approximately 273,790 years forward or backward from 1 January 1970 (21.4.1.1)
requires representing years before 0 or after 9999. ISO 8601 permits expansion of the year
representation, but only by mutual agreement of the partners in information interchange. In the
simplified ECMAScript format, such an expanded year representation shall have 6 digits and is always
prefixed with a + or - sign. The year 0 is considered positive and must be prefixed with a + sign. The
representation of the year 0 as -000000 is invalid. Strings matching the Date Time String
Format with expanded years representing instants in time outside the range of a
time value are
treated as unrecognizable by Date.parse and cause
that function to return NaN without falling back to implementation-specific behaviour
or heuristics.
Note
Examples of date-time values with expanded years:
-271821-04-20T00:00:00Z
271822 B.C.
-000001-01-01T00:00:00Z
2 B.C.
+000000-01-01T00:00:00Z
1 B.C.
+000001-01-01T00:00:00Z
1 A.D.
+001970-01-01T00:00:00Z
1970 A.D.
+002009-12-15T00:00:00Z
2009 A.D.
+275760-09-13T00:00:00Z
275760 A.D.
21.4.1.33 Time Zone Offset String Format
ECMAScript defines a string interchange format for UTC offsets, derived from ISO 8601.
The format is described by the following grammar.
The usage of Unicode code points in this grammar is listed in Table
62.
The abstract operation IsTimeZoneOffsetString takes argument offsetString (a String) and
returns a Boolean. The return value indicates whether offsetString conforms to the grammar
given by UTCOffset. It performs the
following steps when called:
The abstract operation ParseTimeZoneOffsetString takes argument offsetString (a String) and
returns an integer. The return value is the UTC
offset, as a number of nanoseconds, that corresponds to the String offsetString. It performs
the following steps when called:
5. If parsedSign is the single code point U+002D
(HYPHEN-MINUS) or U+2212 (MINUS SIGN), then
a. Let sign be -1.
6. Else,
a. Let sign be 1.
7. NOTE: Applications of StringToNumber below do
not lose precision, since each of the parsed values is guaranteed to be a sufficiently short string
of decimal digits.
is the initial value of the "Date" property of the global
object.
creates and initializes a new Date when called as a constructor.
returns a String representing the current time (UTC) when called as a function rather than as a
constructor.
is a function whose behaviour differs based upon the number and types of its arguments.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit
the specified Date behaviour must include a super call to the Date constructor to create and initialize
the subclass instance with a [[DateValue]] internal slot.
21.4.2.1 Date ( ...values )
This function performs the following steps when called:
1. If NewTarget is undefined, then
a. Let now be the time value
(UTC) identifying the current time.
This function returns the time value
designating the UTC date and time of the occurrence of the call to it.
21.4.3.2 Date.parse ( string )
This function applies the ToString operator to its argument. If
ToString results in an abrupt
completion the Completion
Record is immediately returned. Otherwise, this function interprets the resulting
String as a date and time; it returns a Number, the UTC time
value corresponding to the date and time. The String may be interpreted as a local
time, a UTC time, or a time in some other time zone, depending on the contents of the String. The function
first attempts to parse the String according to the format described in Date Time String Format (21.4.1.32), including
expanded years. If the String does not conform to that format the function may fall back to any
implementation-specific heuristics or implementation-specific date formats. Strings that are
unrecognizable or contain out-of-bounds format element values shall cause this function to return
NaN.
If the String conforms to the Date Time String
Format, substitute values take the place of absent format elements. When the
MM or DD elements are absent, "01" is used. When the
HH, mm, or ss elements are absent, "00" is used.
When the sss element is absent, "000" is used. When the UTC offset
representation is absent, date-only forms are interpreted as a UTC time and date-time forms are
interpreted as a local time.
If x is any Date whose milliseconds amount is zero within a particular implementation of
ECMAScript, then all of the following expressions should produce the same numeric value in that
implementation, if all the properties referenced have their initial values:
is not required to produce the same Number value as the preceding three expressions and, in general, the
value produced by this function is implementation-defined when
given any String value that does not conform to the Date Time String Format (21.4.1.32) and that
could not be produced in that implementation by the toString or toUTCString
method.
This function differs from the Date constructor in two ways: it
returns a time value as a
Number, rather than creating a Date, and it interprets the arguments in UTC rather than as local time.
Unless explicitly defined otherwise, the methods of the Date prototype object defined below are not generic
and the this value passed to them must be an object that has a [[DateValue]] internal slot that has been initialized to a time value.
21.4.4.1 Date.prototype.constructor
The initial value of Date.prototype.constructor is %Date%.
21.4.4.2 Date.prototype.getDate ( )
This method performs the following steps when called:
If month is not present, this method behaves as if month was present with the
value getMonth(). If date is not present, it behaves as if date was
present with the value getDate().
21.4.4.22 Date.prototype.setHours ( hour [ , min [ ,
sec [ , ms ] ] ] )
This method performs the following steps when called:
If min is not present, this method behaves as if min was present with the value
getMinutes(). If sec is not present, it behaves as if sec was
present with the value getSeconds(). If ms is not present, it behaves as if
ms was present with the value getMilliseconds().
21.4.4.23 Date.prototype.setMilliseconds ( ms )
This method performs the following steps when called:
If sec is not present, this method behaves as if sec was present with the value
getSeconds(). If ms is not present, this behaves as if ms was
present with the value getMilliseconds().
21.4.4.25 Date.prototype.setMonth ( month [ , date ] )
This method performs the following steps when called:
If month is not present, this method behaves as if month was present with the
value getUTCMonth(). If date is not present, it behaves as if date
was present with the value getUTCDate().
21.4.4.30 Date.prototype.setUTCHours ( hour [ , min [
, sec [ , ms ] ] ] )
This method performs the following steps when called:
If min is not present, this method behaves as if min was present with the value
getUTCMinutes(). If sec is not present, it behaves as if sec was
present with the value getUTCSeconds(). If ms is not present, it behaves as if
ms was present with the value getUTCMilliseconds().
21.4.4.31 Date.prototype.setUTCMilliseconds ( ms )
This method performs the following steps when called:
If sec is not present, this method behaves as if sec was present with the value
getUTCSeconds(). If ms is not present, it behaves as if ms was
present with the value return by getUTCMilliseconds().
21.4.4.33 Date.prototype.setUTCMonth ( month [ , date
] )
This method performs the following steps when called:
4. If tv is not finite, throw a
RangeError exception.
5. If tv corresponds with a year that cannot be
represented in the Date Time String
Format, throw a RangeError exception.
6. Return a String representation of tv in the Date Time String
Format on the UTC time scale, including all format elements and the UTC offset
representation "Z".
21.4.4.37 Date.prototype.toJSON ( key )
This method provides a String representation of a Date for use by JSON.stringify (25.5.2).
This method is intentionally generic; it does not require that its this value be a
Date. Therefore, it can be transferred to other kinds of objects for use as a method. However, it does
require that any such object have a toISOString method.
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement this
method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the
ECMA-402 API the following specification of this method is used:
This method returns a String value. The contents of the String are implementation-defined, but
are intended to represent the “date” portion of the Date in the current time zone in a convenient,
human-readable form that corresponds to the conventions of the host
environment's current locale.
The meaning of the optional parameters to this method are defined in the ECMA-402 specification;
implementations that do not include ECMA-402 support must not use those parameter positions for anything
else.
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement this
method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the
ECMA-402 API the following specification of this method is used:
This method returns a String value. The contents of the String are implementation-defined, but
are intended to represent the Date in the current time zone in a convenient, human-readable form that
corresponds to the conventions of the host environment's current
locale.
The meaning of the optional parameters to this method are defined in the ECMA-402 specification;
implementations that do not include ECMA-402 support must not use those parameter positions for anything
else.
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement this
method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the
ECMA-402 API the following specification of this method is used:
This method returns a String value. The contents of the String are implementation-defined, but
are intended to represent the “time” portion of the Date in the current time zone in a convenient,
human-readable form that corresponds to the conventions of the host
environment's current locale.
The meaning of the optional parameters to this method are defined in the ECMA-402 specification;
implementations that do not include ECMA-402 support must not use those parameter positions for anything
else.
21.4.4.41 Date.prototype.toString ( )
This method performs the following steps when called:
For any Date d such that d.[[DateValue]] is evenly
divisible by 1000, the result of Date.parse(d.toString()) = d.valueOf(). See
21.4.3.2.
Note 2
This method is not generic; it throws a TypeError exception if its
this value is not a Date. Therefore, it cannot be transferred to other kinds of
objects for use as a method.
21.4.4.41.1 TimeString ( tv )
The abstract operation TimeString takes argument tv (a Number, but not
NaN) and returns a String. It performs the following steps when called:
7. Return the string-concatenation of
weekday, the code unit 0x0020 (SPACE), month, the code unit 0x0020 (SPACE),
day, the code unit 0x0020 (SPACE), yearSign, and paddedYear.
Table 63: Names of days of the week
Number
Name
+0𝔽
"Sun"
1𝔽
"Mon"
2𝔽
"Tue"
3𝔽
"Wed"
4𝔽
"Thu"
5𝔽
"Fri"
6𝔽
"Sat"
Table 64: Names of months of the year
Number
Name
+0𝔽
"Jan"
1𝔽
"Feb"
2𝔽
"Mar"
3𝔽
"Apr"
4𝔽
"May"
5𝔽
"Jun"
6𝔽
"Jul"
7𝔽
"Aug"
8𝔽
"Sep"
9𝔽
"Oct"
10𝔽
"Nov"
11𝔽
"Dec"
21.4.4.41.3 TimeZoneString ( tv )
The abstract operation TimeZoneString takes argument tv (an integral
Number) and returns a
String. It performs the following steps when called:
9. Let tzName be an implementation-defined
string that is either the empty String or the string-concatenation of
the code unit 0x0020 (SPACE), the code unit 0x0028 (LEFT PARENTHESIS), an implementation-defined
timezone name, and the code unit 0x0029 (RIGHT PARENTHESIS).
10. Return the string-concatenation of
offsetSign, offsetHour, offsetMin, and tzName.
21.4.4.41.4 ToDateString ( tv )
The abstract operation ToDateString takes argument tv (an integral Number
or NaN) and returns a String. It performs the following steps when called:
This method returns a String value representing the instant in time corresponding to the
this value. The format of the String is based upon "HTTP-date" from RFC 7231,
generalized to support the full range of times supported by ECMAScript Dates.
11. Return the string-concatenation of
weekday, ",", the code unit 0x0020 (SPACE), day, the code
unit 0x0020 (SPACE), month, the code unit 0x0020 (SPACE), yearSign,
paddedYear, the code unit 0x0020 (SPACE), and TimeString(tv).
21.4.4.44 Date.prototype.valueOf ( )
This method performs the following steps when called:
21.4.4.45 Date.prototype [ @@toPrimitive ] ( hint )
This method is called by ECMAScript language operators to convert a Date to a primitive value. The
allowed values for hint are "default", "number", and
"string". Dates are unique among built-in ECMAScript object in that they treat
"default" as being equivalent to "string", All other built-in
ECMAScript objects treat "default" as being equivalent to "number".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
true }.
The value of the "name" property of this method is
"[Symbol.toPrimitive]".
21.4.5 Properties of Date Instances
Date instances are ordinary objects that inherit
properties from the Date prototype
object. Date instances also have a [[DateValue]] internal slot.
The [[DateValue]] internal slot is the time value
represented by this Date.
is the initial value of the "String" property of the global
object.
creates and initializes a new String object when called as a constructor.
performs a type conversion when called as a function rather than as a constructor.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit
the specified String behaviour must include a super call to the String constructor to create and initialize
the subclass instance with a [[StringData]] internal slot.
22.1.1.1 String ( value )
This function performs the following steps when called:
This function may be called with a variable number of arguments. The first argument is
template and the remainder of the arguments form the Listsubstitutions.
It performs the following steps when called:
1. Let substitutionCount be the number of elements in
substitutions.
This function is intended for use as a tag function of a Tagged Template (13.3.11). When called as
such, the first argument will be a well formed template object and the rest parameter will contain the
substitution values.
Unless explicitly stated otherwise, the methods of the String prototype object defined below are not
generic and the this value passed to them must be either a String value or an object that
has a [[StringData]] internal slot that has been initialized to a String value.
This method returns a single element String containing the code unit at index pos within
the String value resulting from converting this object to a String. If there is no element at that
index, the result is the empty String. The result is a
String value, not a String object.
If pos is an integral Number, then the
result of x.charAt(pos) is equivalent to the result of
x.substring(pos, pos + 1).
This method performs the following steps when called:
5. If position < 0 or position ≥
size, return the empty String.
6. Return the substring of S from
position to position + 1.
Note 2
This method is intentionally generic; it does not require that its this value be a
String object. Therefore, it can be transferred to other kinds of objects for use as a method.
22.1.3.3 String.prototype.charCodeAt ( pos )
Note 1
This method returns a Number (a non-negative integral Number less than
2**16) that is the numeric value of the code unit at index
pos within the String resulting from converting this object to a String. If there is no
element at that index, the result is NaN.
This method performs the following steps when called:
5. If position < 0 or position ≥
size, return NaN.
6. Return the Number value for the numeric
value of the code unit at index position within the String S.
Note 2
This method is intentionally generic; it does not require that its this value be a
String object. Therefore it can be transferred to other kinds of objects for use as a method.
22.1.3.4 String.prototype.codePointAt ( pos )
Note 1
This method returns a non-negative integral Number less than or
equal to 0x10FFFF𝔽 that is the numeric value of the UTF-16 encoded code
point (6.1.4)
starting at the string element at index pos within the String resulting from converting
this object to a String. If there is no element at that index, the result is
undefined. If a valid UTF-16 surrogate pair does not begin
at pos, the result is the code unit at pos.
This method performs the following steps when called:
This method is intentionally generic; it does not require that its this value be a
String object. Therefore it can be transferred to other kinds of objects for use as a method.
22.1.3.5 String.prototype.concat ( ...args )
Note 1
When this method is called it returns the String value consisting of the code units of the
this value (converted to a String) followed by the code units of each of the
arguments converted to a String. The result is a
String value, not a String object.
This method performs the following steps when called:
This method is intentionally generic; it does not require that its this value be a
String object. Therefore it can be transferred to other kinds of objects for use as a method.
22.1.3.6 String.prototype.constructor
The initial value of String.prototype.constructor is %String%.
7. If endPosition is undefined, let
pos be len; else let pos be ? ToIntegerOrInfinity(endPosition).
8. Let end be the result of clampingpos between 0 and len.
9. Let searchLength be the length of
searchStr.
10. If searchLength = 0, return true.
11. Let start be end -
searchLength.
12. If start < 0, return false.
13. Let substring be the substring of
S from start to end.
14. If substring is searchStr, return
true.
15. Return false.
Note 1
This method returns true if the sequence of code units of searchString
converted to a String is the same as the corresponding code units of this object (converted to a
String) starting at endPosition - length(this). Otherwise it returns
false.
Note 2
Throwing an exception if the first argument is a RegExp is specified in order to allow future
editions to define extensions that allow such argument values.
Note 3
This method is intentionally generic; it does not require that its this value be a
String object. Therefore, it can be transferred to other kinds of objects for use as a method.
22.1.3.8 String.prototype.includes ( searchString [ ,
position ] )
This method performs the following steps when called:
7. Assert: If position is
undefined, then pos is 0.
8. Let len be the length of S.
9. Let start be the result of clampingpos between 0 and len.
10. Let index be StringIndexOf(S,
searchStr, start).
11. If index ≠ -1, return true.
12. Return false.
Note 1
If searchString appears as a substring of the result of
converting this object to a String, at one or more indices that are greater than or equal to
position, this function returns true; otherwise, it returns
false. If position is undefined, 0 is assumed, so as
to search all of the String.
Note 2
Throwing an exception if the first argument is a RegExp is specified in order to allow future
editions to define extensions that allow such argument values.
Note 3
This method is intentionally generic; it does not require that its this value be a
String object. Therefore, it can be transferred to other kinds of objects for use as a method.
22.1.3.9 String.prototype.indexOf ( searchString [ ,
position ] )
Note 1
If searchString appears as a substring of the result of
converting this object to a String, at one or more indices that are greater than or equal to
position, then the smallest such index is returned; otherwise,
-1𝔽 is returned. If position is
undefined, +0𝔽 is assumed, so as to search all of
the String.
This method performs the following steps when called:
This method is intentionally generic; it does not require that its this value be a
String object. Therefore, it can be transferred to other kinds of objects for use as a method.
22.1.3.10 String.prototype.isWellFormed ( )
This method performs the following steps when called:
22.1.3.11 String.prototype.lastIndexOf ( searchString [ ,
position ] )
Note 1
If searchString appears as a substring of the result of
converting this object to a String at one or more indices that are smaller than or equal to
position, then the greatest such index is returned; otherwise,
-1𝔽 is returned. If position is
undefined, the length of the String value is assumed, so as to search all of the
String.
This method performs the following steps when called:
This method is intentionally generic; it does not require that its this value be a
String object. Therefore, it can be transferred to other kinds of objects for use as a method.
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement this
method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the
ECMA-402 API the following specification of this method is used:
This method returns a Number other than NaN representing the result of an implementation-defined
locale-sensitive String comparison of the this value (converted to a String
S) with that (converted to a String thatValue). The result is intended to
correspond with a sort order of String values according
to conventions of the host environment's current
locale, and will be negative when S is ordered before thatValue, positive when
S is ordered after thatValue, and zero in all other cases (representing no relative
ordering between S and thatValue).
Before performing the comparisons, this method performs the following steps to prepare the Strings:
The meaning of the optional second and third parameters to this method are defined in the ECMA-402
specification; implementations that do not include ECMA-402 support must not assign any other
interpretation to those parameter positions.
The actual return values are implementation-defined to
permit encoding additional information in them, but this method, when considered as a method of two
arguments, is required to be a consistent comparator
defining a total ordering on the set of all Strings. This method is also required to recognize and honour
canonical equivalence according to the Unicode Standard, including returning
+0𝔽 when comparing distinguishable Strings that are canonically equivalent.
Note 1
This method itself is not directly suitable as an argument to Array.prototype.sort
because the latter requires a function of two arguments.
Note 2
This method may rely on whatever language- and/or locale-sensitive comparison functionality is
available to the ECMAScript environment from the host environment, and is
intended to compare according to the conventions of the host
environment's current locale. However, regardless of comparison capabilities, this
method must recognize and honour canonical equivalence according to the Unicode Standard—for example,
the following comparisons must all return +0𝔽:
// Å ANGSTROM SIGN vs.// Å LATIN CAPITAL LETTER A + COMBINING RING ABOVE"\u212B".localeCompare("A\u030A")
// Ω OHM SIGN vs.// Ω GREEK CAPITAL LETTER OMEGA"\u2126".localeCompare("\u03A9")
// ṩ LATIN SMALL LETTER S WITH DOT BELOW AND DOT ABOVE vs.// ṩ LATIN SMALL LETTER S + COMBINING DOT ABOVE + COMBINING DOT BELOW"\u1E69".localeCompare("s\u0307\u0323")
// ḍ̇ LATIN SMALL LETTER D WITH DOT ABOVE + COMBINING DOT BELOW vs.// ḍ̇ LATIN SMALL LETTER D WITH DOT BELOW + COMBINING DOT ABOVE"\u1E0B\u0323".localeCompare("\u1E0D\u0307")
// 가 HANGUL CHOSEONG KIYEOK + HANGUL JUNGSEONG A vs.// 가 HANGUL SYLLABLE GA"\u1100\u1161".localeCompare("\uAC00")
It is recommended that this method should not honour Unicode compatibility equivalents or
compatibility decompositions as defined in the Unicode Standard, chapter 3, section 3.7.
Note 3
This method is intentionally generic; it does not require that its this value be a
String object. Therefore, it can be transferred to other kinds of objects for use as a method.
22.1.3.13 String.prototype.match ( regexp )
This method performs the following steps when called:
This method is intentionally generic; it does not require that its this value be a
String object. Therefore, it can be transferred to other kinds of objects for use as a method.
22.1.3.14 String.prototype.matchAll ( regexp )
This method performs a regular expression match of the String representing the this
value against regexp and returns an iterator. Each iteration result's value is an Array
containing the results of the match, or null if the String did not match.
This method is intentionally generic, it does not require that its
this value be a String object. Therefore, it can be transferred to other kinds of
objects for use as a method.
Note 2
Similarly to String.prototype.split,
String.prototype.matchAll is designed to typically act without mutating its inputs.
22.1.3.15 String.prototype.normalize ( [ form ] )
This method performs the following steps when called:
This method is intentionally generic; it does not require that its this value be a
String object. Therefore it can be transferred to other kinds of objects for use as a method.
The abstract operation StringPad takes arguments S (a String), maxLength (a
non-negative integer), fillString (a
String), and placement (start or end) and
returns a String. It performs the following steps when called:
1. Let stringLength be the length of S.
2. If maxLength ≤ stringLength, return
S.
3. If fillString is the empty String, return
S.
4. Let fillLen be maxLength -
stringLength.
5. Let truncatedStringFiller be the String value
consisting of repeated concatenations of fillString truncated to length
fillLen.
6. If placement is start, return
the string-concatenation of
truncatedStringFiller and S.
The argument maxLength will be clamped such that it can be no smaller than the length of
S.
Note 2
The argument fillString defaults to " " (the String value consisting
of the code unit 0x0020 SPACE).
22.1.3.17.3 ToZeroPaddedDecimalString ( n, minLength
)
The abstract operation ToZeroPaddedDecimalString takes arguments n (a non-negative integer)
and minLength (a
non-negative integer) and returns a String. It
performs the following steps when called:
1. Let S be the String representation of n,
formatted as a decimal number.
4. If n < 0 or n = +∞, throw a
RangeError exception.
5. If n = 0, return the empty String.
6. Return the String value that is made from n copies of
S appended together.
Note 1
This method creates the String value consisting of the code units of the this
value (converted to String) repeated count times.
Note 2
This method is intentionally generic; it does not require that its this value be a
String object. Therefore, it can be transferred to other kinds of objects for use as a method.
This method is intentionally generic; it does not require that its this value be a
String object. Therefore, it can be transferred to other kinds of objects for use as a method.
The abstract operation GetSubstitution takes arguments matched (a String), str (a
String), position (a non-negative integer), captures (a
List of
either Strings or undefined), namedCaptures (an Object or
undefined), and replacementTemplate (a String) and returns either a
normal completion
containing a String or a throw
completion. For the purposes of this abstract operation, a decimal digit is
a code unit in the inclusive interval from
0x0030 (DIGIT ZERO) to 0x0039 (DIGIT NINE). It performs the following steps when called:
5. Repeat, while templateRemainder is not the empty
String,
a. NOTE: The following steps
isolate ref (a prefix of templateRemainder), determine
refReplacement (its replacement), and then append that replacement to
result.
b. If templateRemainder starts with
"$$", then
i. Let ref be "$$".
ii. Let refReplacement be
"$".
c. Else if templateRemainder starts with
"$`", then
i. Let ref be "$`".
ii. Let refReplacement be the substring of
str from 0 to position.
d. Else if templateRemainder starts with
"$&", then
i. Let ref be "$&".
ii. Let refReplacement be matched.
e. Else if templateRemainder starts with
"$'" (0x0024 (DOLLAR SIGN) followed by 0x0027 (APOSTROPHE)), then
i. Let ref be "$'".
ii. Let matchLength be the length of
matched.
iii. Let tailPos be position +
matchLength.
iv. Let refReplacement be the substring of
str from min(tailPos,
stringLength).
v. NOTE: tailPos can exceed
stringLength only if this abstract operation was invoked by a call to the
intrinsic @@replace
method of %RegExp.prototype%
on an object whose "exec" property is not the intrinsic
%RegExp.prototype.exec%.
f. Else if templateRemainder starts with
"$" followed by 1 or more decimal digits, then
i. If templateRemainder starts with
"$" followed by 2 or more decimal digits, let digitCount be 2.
Otherwise, let digitCount be 1.
ii. Let digits be the substring of
templateRemainder from 1 to 1 + digitCount.
v. Let captureLen be the number of elements in
captures.
vi. If index > captureLen and
digitCount = 2, then
1. NOTE: When a two-digit replacement pattern
specifies an index exceeding the count of capturing groups, it is treated as a one-digit
replacement pattern followed by a literal digit.
2. Set digitCount to 1.
3. Set digits to the substring of
digits from 0 to 1.
This method is intentionally generic; it does not require that its this value be a
String object. Therefore, it can be transferred to other kinds of objects for use as a method.
22.1.3.22 String.prototype.slice ( start, end )
This method returns a substring of the result of converting this object to a
String, starting from index start and running to, but not including, index end (or
through the end of the String if end is undefined). If start is
negative, it is treated as sourceLength + start
where sourceLength is the length of the String. If end is negative, it is treated as
sourceLength + end where sourceLength
is the length of the String. The result is a
String value, not a String object.
This method is intentionally generic; it does not require that its this value be a
String object. Therefore it can be transferred to other kinds of objects for use as a method.
This method returns an Array into which substrings of the result of converting this object to a String
have been stored. The substrings are determined by searching from left to right for occurrences of
separator; these occurrences are not part of any String in the returned array, but serve to
divide up the String value. The value of separator may be a String of any length or it may be
an object, such as a RegExp, that has a @@split method.
The value of separator may be an empty String. In this case, separator does not
match the empty substring at the beginning or end of the input String, nor
does it match the empty substring at the end of the previous separator
match. If separator is the empty String, the String is split up into individual code unit
elements; the length of the result array equals the length of the String, and each
substring contains one code unit.
If the this value is (or converts to) the empty String, the result depends on
whether separator can match the empty String. If it can, the result array contains no
elements. Otherwise, the result array contains one element, which is the empty String.
If separator is undefined, then the result array contains just one
String, which is the this value (converted to a String). If limit is not
undefined, then the output array is truncated so that it contains no more than
limit elements.
Note 2
This method is intentionally generic; it does not require that its this value be a
String object. Therefore, it can be transferred to other kinds of objects for use as a method.
22.1.3.24 String.prototype.startsWith ( searchString [ ,
position ] )
This method performs the following steps when called:
7. If position is undefined, let
pos be 0; else let pos be ? ToIntegerOrInfinity(position).
8. Let start be the result of clampingpos between 0 and len.
9. Let searchLength be the length of
searchStr.
10. If searchLength = 0, return true.
11. Let end be start +
searchLength.
12. If end > len, return
false.
13. Let substring be the substring of
S from start to end.
14. If substring is searchStr, return
true.
15. Return false.
Note 1
This method returns true if the sequence of code units of searchString
converted to a String is the same as the corresponding code units of this object (converted to a
String) starting at index position. Otherwise it returns false.
Note 2
Throwing an exception if the first argument is a RegExp is specified in order to allow future
editions to define extensions that allow such argument values.
Note 3
This method is intentionally generic; it does not require that its this value be a
String object. Therefore, it can be transferred to other kinds of objects for use as a method.
22.1.3.25 String.prototype.substring ( start, end )
This method returns a substring of the result of converting this object to a
String, starting from index start and running to, but not including, index end of
the String (or through the end of the String if end is undefined). The
result is a
String value, not a String object.
If either argument is NaN or negative, it is replaced with zero; if either argument is
strictly greater than the length of the String, it is replaced with the length of the String.
If start is strictly greater than end, they are swapped.
This method is intentionally generic; it does not require that its this value be a
String object. Therefore, it can be transferred to other kinds of objects for use as a method.
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement this
method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the
ECMA-402 API the following specification of this method is used:
This method interprets a String value as a sequence of UTF-16 encoded code points, as described in
6.1.4.
It works exactly the same as toLowerCase except that it is intended to yield a
locale-sensitive result corresponding with conventions of the host
environment's current locale. There will only be a difference in the few cases (such as
Turkish) where the rules for that language conflict with the regular Unicode case mappings.
The meaning of the optional parameters to this method are defined in the ECMA-402 specification;
implementations that do not include ECMA-402 support must not use those parameter positions for anything
else.
Note
This method is intentionally generic; it does not require that its this value be a
String object. Therefore, it can be transferred to other kinds of objects for use as a method.
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement this
method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the
ECMA-402 API the following specification of this method is used:
This method interprets a String value as a sequence of UTF-16 encoded code points, as described in
6.1.4.
It works exactly the same as toUpperCase except that it is intended to yield a
locale-sensitive result corresponding with conventions of the host
environment's current locale. There will only be a difference in the few cases (such as
Turkish) where the rules for that language conflict with the regular Unicode case mappings.
The meaning of the optional parameters to this method are defined in the ECMA-402 specification;
implementations that do not include ECMA-402 support must not use those parameter positions for anything
else.
Note
This method is intentionally generic; it does not require that its this value be a
String object. Therefore, it can be transferred to other kinds of objects for use as a method.
22.1.3.28 String.prototype.toLowerCase ( )
This method interprets a String value as a sequence of UTF-16 encoded code points, as described in
6.1.4.
The result must be derived according to the locale-insensitive case mappings in the Unicode Character
Database (this explicitly includes not only the file UnicodeData.txt, but
also all locale-insensitive mappings in the file SpecialCasing.txt
that accompanies it).
Note 1
The case mapping of some code points may produce multiple code points. In this case the result String
may not be the same length as the source String. Because both toUpperCase and
toLowerCase have context-sensitive behaviour, the methods are not symmetrical. In other
words, s.toUpperCase().toLowerCase() is not necessarily equal to
s.toLowerCase().
Note 2
This method is intentionally generic; it does not require that its this value be a
String object. Therefore, it can be transferred to other kinds of objects for use as a method.
22.1.3.29 String.prototype.toString ( )
This method performs the following steps when called:
For a String object, this method happens to return the same thing as the valueOf method.
22.1.3.30 String.prototype.toUpperCase ( )
This method interprets a String value as a sequence of UTF-16 encoded code points, as described in
6.1.4.
It behaves in exactly the same way as String.prototype.toLowerCase, except that the String
is mapped using the toUppercase algorithm of the Unicode Default Case Conversion.
Note
This method is intentionally generic; it does not require that its this value be a
String object. Therefore, it can be transferred to other kinds of objects for use as a method.
This method is intentionally generic; it does not require that its this value be a
String object. Therefore, it can be transferred to other kinds of objects for use as a method.
22.1.3.32.1 TrimString ( string, where )
The abstract operation TrimString takes arguments string (an ECMAScript language
value) and where (start,
end, or start+end) and returns either a normal completion
containing a String or a throw
completion. It interprets string as a sequence of UTF-16 encoded code
points, as described in 6.1.4.
It performs the following steps when called:
b. Let T be the String value that is a copy of
S with both leading and trailing white space removed.
6. Return T.
The definition of white space is the union of WhiteSpace and LineTerminator.
When
determining whether a Unicode code point is in Unicode general category “Space_Separator” (“Zs”), code
unit sequences are interpreted as UTF-16 encoded code point sequences as specified in 6.1.4.
22.1.3.33 String.prototype.trimEnd ( )
This method interprets a String value as a sequence of UTF-16 encoded code points, as described in
6.1.4.
This method is intentionally generic; it does not require that its this value be a
String object. Therefore, it can be transferred to other kinds of objects for use as a method.
22.1.3.34 String.prototype.trimStart ( )
This method interprets a String value as a sequence of UTF-16 encoded code points, as described in
6.1.4.
This method is intentionally generic; it does not require that its this value be a
String object. Therefore, it can be transferred to other kinds of objects for use as a method.
22.1.3.35 String.prototype.valueOf ( )
This method performs the following steps when called:
The value of the "name" property of this method is
"[Symbol.iterator]".
22.1.4 Properties of String Instances
String instances are String exotic objects and have
the internal methods specified for such objects. String instances inherit properties from the String prototype
object. String instances also have a [[StringData]] internal
slot. The [[StringData]] internal slot is the String value represented by this
String object.
String instances have a "length" property, and a set of enumerable properties with
integer-indexed names.
22.1.4.1 length
The number of elements in the String value represented by this String object.
Once a String object is initialized, this property is unchanging. It has the attributes { [[Writable]]: false, [[Enumerable]]:
false, [[Configurable]]: false }.
22.1.5 String Iterator Objects
A String Iterator is an object, that represents a specific iteration over some specific String instance
object. There is not a named constructor for String Iterator objects.
Instead, String iterator objects are created by calling certain methods of String instance objects.
22.1.5.1 The %StringIteratorPrototype% Object
The %StringIteratorPrototype% object:
has properties that are inherited by all String Iterator Objects.
The initial value of the @@toStringTag property is
the String value "String Iterator".
This property has the attributes { [[Writable]]: false,
[[Enumerable]]: false, [[Configurable]]: true }.
22.2 RegExp (Regular Expression) Objects
A RegExp object contains a regular expression and the associated flags.
Note
The form and functionality of regular expressions is modelled after the regular expression facility in
the Perl 5 programming language.
22.2.1 Patterns
The RegExp constructor applies the following
grammar to the input pattern String. An error occurs if the grammar cannot interpret the String as an
expansion of Pattern.
The abstract operation CountLeftCapturingParensWithin takes argument node (a Parse Node) and returns a
non-negative integer. It returns the number of
left-capturing parentheses in node. A left-capturing parenthesis is any ( pattern character that is matched
by the ( terminal of the Atom::(GroupSpecifieroptDisjunction) production.
The abstract operation CountLeftCapturingParensBefore takes argument node (a Parse Node) and returns a
non-negative integer. It returns the number of
left-capturing
parentheses within the enclosing pattern that occur to the left of node.
The abstract operation GroupSpecifiersThatMatch takes argument thisGroupName (a GroupNameParse Node) and returns a
List of
GroupSpecifierParse Nodes. It performs the
following steps when called:
The syntax-directed
operation RegExpIdentifierCodePoints takes no arguments and returns a List of
code points. It is defined piecewise over the following productions:
The syntax-directed
operation RegExpIdentifierCodePoint takes no arguments and returns a code point. It is
defined piecewise over the following productions:
A regular expression pattern is converted into an Abstract Closure using the
process described below. An implementation is encouraged to use more efficient algorithms than the ones
listed below, as long as the results are the same. The Abstract
Closure is used as the value of a RegExp object's [[RegExpMatcher]] internal slot.
A Pattern is a
BMP pattern if its associated flags contain neither a u nor a v. Otherwise, it is
a Unicode pattern. A BMP pattern matches against a String interpreted as consisting of a sequence of 16-bit
values that are Unicode code points in the range of the Basic Multilingual Plane. A Unicode pattern matches
against a String interpreted as consisting of Unicode code points encoded using UTF-16. In the context of
describing the behaviour of a BMP pattern “character” means a single 16-bit Unicode BMP code point. In the
context of describing the behaviour of a Unicode pattern “character” means a UTF-16 encoded code point
(6.1.4). In
either context, “character value” means the numeric value of the corresponding non-encoded code point.
The syntax and semantics of Pattern is defined as if the source text
for the Pattern
was a List of
SourceCharacter values where each
SourceCharacter corresponds to a
Unicode code point. If a BMP pattern contains a non-BMP SourceCharacter the entire
pattern is encoded using UTF-16 and the individual code units of that encoding are used as the elements of
the List.
Note
For example, consider a pattern expressed in source text as the single non-BMP character U+1D11E
(MUSICAL SYMBOL G CLEF). Interpreted as a Unicode pattern, it would be a single element (character)
List
consisting of the single code point U+1D11E. However, interpreted as a BMP pattern, it is first UTF-16
encoded to produce a two element List
consisting of the code units 0xD834 and 0xDD1E.
Patterns are passed to the RegExp constructor as ECMAScript String
values in which non-BMP characters are UTF-16 encoded. For example, the single character MUSICAL SYMBOL
G CLEF pattern, expressed as a String value, is a
String of length 2 whose elements were the code units 0xD834 and 0xDD1E. So no
further translation of the string would be necessary to process it as a BMP pattern consisting of two
pattern characters. However, to process it as a Unicode pattern UTF16SurrogatePairToCodePoint
must be used in producing a List
whose sole element is a single pattern character, the code point U+1D11E.
An implementation may not actually perform such translations to or from UTF-16, but the semantics of
this specification requires that the result of pattern matching be as if such translations were
performed.
22.2.2.1 Notation
The descriptions below use the following internal data structures:
A CharSetElement is one of the two following entities:
If rer.[[UnicodeSets]] is false, then a
CharSetElement is a character in the sense of the Pattern Semantics above.
If rer.[[UnicodeSets]] is true, then a
CharSetElement is a sequence whose elements are characters in the sense of the Pattern Semantics
above. This includes the empty sequence, sequences of one character, and sequences of more than one
character. For convenience, when working with CharSetElements of this kind, an individual character
is treated interchangeably with a sequence of one character.
A CharSet is a mathematical set of
CharSetElements.
A CaptureRange is a
Record {
[[StartIndex]], [[EndIndex]] } that represents the
range of characters included in a capture, where [[StartIndex]] is an integer
representing the start index (inclusive) of the range within Input, and [[EndIndex]] is an integer representing the end index
(exclusive) of the range within Input. For any CaptureRange,
these indices
must satisfy the invariant that [[StartIndex]] ≤ [[EndIndex]].
A MatchState is a Record {
[[Input]], [[EndIndex]], [[Captures]] } where [[Input]] is a List of
characters representing the String being matched, [[EndIndex]] is an integer,
and [[Captures]] is a List of
values, one for each left-capturing
parenthesis in the pattern. States are used to represent partial match states in the
regular expression matching algorithms. The [[EndIndex]] is one plus the index
of the last input character matched so far by the pattern, while [[Captures]]
holds the results of capturing parentheses. The nth element of [[Captures]] is either a CaptureRange representing
the range of characters captured by the nth set of capturing parentheses, or
undefined if the nth set of capturing parentheses hasn't been
reached yet. Due to backtracking, many States may be in use at any time during the matching process.
A MatchResult is either a
MatchState or the special
token failure that indicates that the match failed.
A MatcherContinuation is an Abstract
Closure that takes one MatchState argument and
returns a MatchResult result. The
MatcherContinuation
attempts to match the remaining portion (specified by the closure's captured values) of the pattern
against Input, starting at the intermediate state given by its MatchState argument. If the
match succeeds, the MatcherContinuation
returns the final MatchState that it reached;
if the match fails, the MatcherContinuation
returns failure.
A Matcher is an Abstract Closure that takes
two arguments—a MatchState and a MatcherContinuation—and
returns a MatchResult result. A
Matcher attempts to match a
middle subpattern (specified by the closure's captured values) of the pattern against the MatchState's [[Input]], starting at the intermediate state given by its MatchState argument. The
MatcherContinuation
argument should be a closure that matches the rest of the pattern. After matching the subpattern of a
pattern to obtain a new MatchState, the Matcher then calls MatcherContinuation
on that new MatchState to test if the
rest of the pattern can match as well. If it can, the Matcher returns
the MatchState returned by
MatcherContinuation;
if not, the Matcher may try different
choices at its choice points, repeatedly calling MatcherContinuation
until it either succeeds or all possibilities have been exhausted.
22.2.2.1.1 RegExp Records
A RegExp Record is a Record
value used to store information about a RegExp that is needed during compilation and possibly during
matching.
d. Let cap be a List
of rer.[[CapturingGroupsCount]]undefined
values, indexed 1 through rer.[[CapturingGroupsCount]].
e. Let x be the MatchState
{ [[Input]]: Input, [[EndIndex]]:
index, [[Captures]]: cap }.
f. Return m(x, c).
Note
A Pattern compiles to an Abstract Closure value.
RegExpBuiltinExec can
then apply this procedure to a List of
characters and an offset within that List to
determine whether the pattern would match starting at exactly that offset within the List,
and, if it does match, what the values of the capturing parentheses would be. The algorithms in
22.2.2 are designed so
that compiling a pattern may throw a SyntaxError exception; on the other hand, once
the pattern is successfully compiled, applying the resulting Abstract
Closure to find a match in a List of
characters cannot throw an exception (except for any implementation-defined
exceptions that can occur anywhere such as out-of-memory).
The | regular expression operator separates two alternatives. The pattern first tries to
match the left Alternative (followed by the
sequel of the regular expression); if it fails, it tries to match the right Disjunction (followed by the
sequel of the regular expression). If the left Alternative, the right Disjunction,
and the sequel all have choice points, all choices in the sequel are tried before moving on to the
next choice in the left Alternative. If choices in the
left Alternative are exhausted, the
right Disjunction is tried instead of
the left Alternative. Any capturing
parentheses inside a portion of the pattern skipped by | produce
undefined values instead of Strings. Thus, for example,
Consecutive Terms
try to simultaneously match
consecutive portions of Input. When direction is forward,
if the left Alternative, the right Term, and the sequel
of the regular expression all have choice points, all choices in the sequel are tried before moving on
to the next choice in the right Term, and all choices in the right
Term are tried
before moving on to the next choice in the left Alternative. When
direction is backward, the evaluation order of Alternative
and Term are
reversed.
The abstract operation RepeatMatcher takes arguments m (a Matcher),
min (a non-negative integer), max (a non-negative
integer or +∞), greedy (a
Boolean), x (a MatchState), c (a
MatcherContinuation),
parenIndex (a non-negative integer), and parenCount (a
non-negative integer) and returns a MatchResult. It performs the
following steps when called:
1. If max = 0, return c(x).
2. Let d be a new MatcherContinuation
with parameters (y) that captures m, min, max,
greedy, x, c, parenIndex, and parenCount and
performs the following steps when called:
b. If min = 0 and
y.[[EndIndex]] = x.[[EndIndex]], return failure.
c. If min = 0, let min2 be 0; otherwise
let min2 be min - 1.
d. If max = +∞, let max2 be +∞;
otherwise let max2 be max - 1.
e. Return RepeatMatcher(m,
min2, max2, greedy, y, c,
parenIndex, parenCount).
3. Let cap be a copy of x.[[Captures]].
4. For each integerk in the
inclusive interval from
parenIndex + 1 to parenIndex + parenCount, set
cap[k] to undefined.
5. Let Input be x.[[Input]].
6. Let e be x.[[EndIndex]].
7. Let xr be the MatchState {
[[Input]]: Input, [[EndIndex]]:
e, [[Captures]]: cap }.
8. If min ≠ 0, return m(xr,
d).
9. If greedy is false, then
a. Let z be c(x).
b. If z is not failure,
return z.
c. Return m(xr, d).
10. Let z be m(xr, d).
11. If z is not failure, return
z.
12. Return c(x).
Note 1
An Atom
followed by a Quantifier is repeated the
number of times specified by the Quantifier. A Quantifier
can be non-greedy, in which case the Atom pattern is repeated as few
times as possible while still matching the sequel, or it can be greedy, in which case the Atom pattern is
repeated as many times as possible while still matching the sequel. The Atom pattern is repeated rather than
the input character sequence that it matches, so different repetitions of the Atom can match
different input substrings.
Note 2
If the Atom
and the sequel of the regular expression all have choice points, the Atom is first matched as many (or as
few, if non-greedy) times as possible. All choices in the sequel are tried before moving on to the
next choice in the last repetition of Atom. All choices in the last
(nth) repetition of Atom are tried before moving on to
the next choice in the next-to-last (n - 1)st repetition of Atom; at which point it may turn out
that more or fewer repetitions of Atom are now possible; these are
exhausted (again, starting with either as few or as many as possible) before moving on to the next
choice in the (n - 1)st repetition of Atom and so on.
Compare
/a[a-z]{2,4}/.exec("abcdefghi")
which returns "abcde" with
/a[a-z]{2,4}?/.exec("abcdefghi")
which returns "abc".
Consider also
/(aa|aabaac|ba|b|c)*/.exec("aabaac")
which, by the choice point ordering above, returns the array
["aaba", "ba"]
and not any of:
["aabaac", "aabaac"]
["aabaac", "c"]
The above ordering of choice points can be used to write a regular expression that calculates the
greatest common divisor of two numbers (represented in unary notation). The following example
calculates the gcd of 10 and 15:
Step 4 of the
RepeatMatcher clears Atom's captures each time Atom is repeated.
We can see its behaviour in the regular expression
/(z)((a+)?(b+)?(c))*/.exec("zaacbbbcac")
which returns the array
["zaacbbbcac", "z", "ac", "a", undefined, "c"]
and not
["zaacbbbcac", "z", "ac", "a", "bbb", "c"]
because each iteration of the outermost * clears all captured Strings contained in the
quantified Atom,
which in this case includes
capture Strings numbered 2, 3, 4, and 5.
Note 4
Step 2.b of the
RepeatMatcher states that once the minimum number of repetitions has been satisfied, any more
expansions of Atom that match the empty character
sequence are not considered for further repetitions. This prevents the regular expression engine
from falling into an infinite loop on patterns such as:
/(a*)*/.exec("b")
or the slightly more complicated:
/(a*)b\1+/.exec("baaaac")
which returns the array
["b", ""]
22.2.2.3.2 EmptyMatcher ( )
The abstract operation EmptyMatcher takes no arguments and returns a Matcher. It
performs the following steps when called:
1. Return a new Matcher
with parameters (x, c) that captures nothing and performs the following steps
when called:
The abstract operation MatchTwoAlternatives takes arguments m1 (a Matcher) and m2 (a
Matcher) and returns a Matcher. It performs the
following steps when called:
1. Return a new Matcher
with parameters (x, c) that captures m1 and m2 and
performs the following steps when called:
The abstract operation MatchSequence takes arguments m1 (a Matcher),
m2 (a Matcher), and
direction (forward or backward) and returns a
Matcher. It performs the
following steps when called:
1. If direction is forward, then
a. Return a new Matcher
with parameters (x, c) that captures m1 and m2 and
performs the following steps when called:
e. If e = 0, or if rer.[[Multiline]] is true and the character
Input[e - 1] is matched by LineTerminator, then
i. Return c(x).
f. Return failure.
Note 2
Even when the y flag is used with a pattern, ^ always matches only at the
beginning of Input, or (if rer.[[Multiline]] is
true) at the beginning of a line.
j. Let z be the MatchState
{ [[Input]]: Input, [[EndIndex]]:
xe, [[Captures]]: cap }.
k. Return c(z).
Note 3
The form (?=Disjunction)
specifies a zero-width positive lookahead. In order for it to succeed, the pattern inside Disjunction
must match at the current position, but the current position is not advanced before matching the
sequel. If Disjunction can match at the
current position in several ways, only the first one is tried. Unlike other regular expression
operators, there is no backtracking into a (?= form (this unusual behaviour is inherited
from Perl). This only matters when the Disjunction contains capturing
parentheses and the sequel of the pattern contains backreferences to those captures.
For example,
/(?=(a+))/.exec("baaabac")
matches the empty String immediately after the first b and therefore returns the array:
["", "aaa"]
To illustrate the lack of backtracking into the lookahead, consider:
The form (?!Disjunction)
specifies a zero-width negative lookahead. In order for it to succeed, the pattern inside Disjunction
must fail to match at the current position. The current position is not advanced before matching the
sequel. Disjunction can contain
capturing parentheses, but backreferences to them only make sense from within Disjunction
itself. Backreferences to these capturing parentheses from elsewhere in the pattern always return
undefined because the negative lookahead must fail for the pattern to succeed. For
example,
/(.*?)a(?!(a+)b\2c)\2(.*)/.exec("baaabaac")
looks for an a not immediately followed by some positive number n of a's, a
b, another n a's (specified by the first \2) and a
c. The second \2 is outside the negative lookahead, so it matches against
undefined and therefore always succeeds. The whole expression returns the array:
The abstract operation IsWordChar takes arguments rer (a RegExp Record),
Input (a List of
characters), and e (an integer) and returns a Boolean. It
performs the following steps when called:
1. Let InputLength be the number of elements in
Input.
The syntax-directed
operation CompileQuantifier takes no arguments and returns a Record with
fields [[Min]] (a non-negative integer), [[Max]]
(a non-negative integer or +∞), and [[Greedy]] (a Boolean). It is defined piecewise over the following productions:
The syntax-directed
operation CompileQuantifierPrefix takes no arguments and returns a Record with
fields [[Min]] (a non-negative integer) and [[Max]]
(a non-negative integer or +∞). It is defined piecewise
over the following productions:
3. If rer.[[UnicodeSets]] is
false, or if every CharSetElement of
cs consists of a single character (including if cs is empty), return CharacterSetMatcher(rer,
cs, cc.[[Invert]], direction).
3. Let r be the CaptureRange
{ [[StartIndex]]: ye, [[EndIndex]]: xe }.
viii. Set cap[parenIndex + 1] to
r.
ix. Let z be the MatchState { [[Input]]: Input, [[EndIndex]]:
ye, [[Captures]]: cap }.
x. Return c(z).
d. Return m(x, d).
Note 2
Parentheses of the form (Disjunction)
serve both to group the components of the Disjunction pattern together
and to save the result of the match. The result can be used either in a backreference (\
followed by a non-zero decimal number), referenced in a replace String, or returned as part of an
array from the regular expression matching Abstract Closure. To
inhibit the capturing behaviour of parentheses, use the form (?:Disjunction) instead.
An escape sequence of the form \ followed by a non-zero decimal number n
matches the result of the nth set of capturing parentheses (22.2.2.1). It is an error
if the regular expression has fewer than n capturing parentheses. If the regular expression
has n or more capturing parentheses but the nth one is
undefined because it has not captured anything, then the backreference always
succeeds.
2. If rer.[[UnicodeSets]] is
false, or if every CharSetElement of
cs consists of a single character (including if cs is empty), return CharacterSetMatcher(rer,
cs, false, direction).
22.2.2.7.1 CharacterSetMatcher ( rer, A,
invert, direction )
The abstract operation CharacterSetMatcher takes arguments rer (a RegExp Record), A
(a CharSet), invert (a
Boolean), and direction (forward or backward)
and returns a Matcher. It performs the
following steps when called:
l. If there exists a CharSetElement in
A containing exactly one character a such that Canonicalize(rer,
a) is cc, let found be true. Otherwise, let
found be false.
m. If invert is false and
found is false, return failure.
n. If invert is true and
found is true, return failure.
o. Let cap be x.[[Captures]].
p. Let y be the MatchState { [[Input]]: Input, [[EndIndex]]:
f, [[Captures]]: cap }.
q. Return c(y).
22.2.2.7.2 BackreferenceMatcher ( rer, n,
direction )
The abstract operation BackreferenceMatcher takes arguments rer (a RegExp Record), n
(a positive integer), and direction
(forward or backward) and returns a Matcher. It performs the
following steps when called:
p. If there exists an integeri in the interval from 0 (inclusive) to
len (exclusive) such that Canonicalize(rer,
Input[rs + i]) is not Canonicalize(rer,
Input[g + i]), return failure.
q. Let y be the MatchState { [[Input]]: Input, [[EndIndex]]:
f, [[Captures]]: cap }.
r. Return c(y).
22.2.2.7.3 Canonicalize ( rer, ch )
The abstract operation Canonicalize takes arguments rer (a RegExp Record) and
ch (a character) and returns a character. It performs the following steps when called:
a. If the file CaseFolding.txt
of the Unicode Character Database provides a simple or common case folding mapping for
ch, return the result of applying that mapping to ch.
9. If the numeric value of ch ≥ 128 and the numeric
value of cu < 128, return ch.
10. Return cu.
Note
In case-insignificant matches when HasEitherUnicodeFlag(rer)
is true, all characters are implicitly case-folded using the simple mapping
provided by the Unicode Standard immediately before they are compared. The simple mapping always
maps to a single code point, so it does not map, for example, ß (U+00DF LATIN SMALL
LETTER SHARP S) to ss or SS. It may however map code points outside the
Basic Latin block to code points within it—for example, ſ (U+017F LATIN SMALL LETTER
LONG S) case-folds to s (U+0073 LATIN SMALL LETTER S) and K (U+212A KELVIN
SIGN) case-folds to k (U+006B LATIN SMALL LETTER K). Strings containing those code
points are matched by regular expressions such as /[a-z]/ui.
In case-insignificant matches when HasEitherUnicodeFlag(rer)
is false, the mapping is based on Unicode Default Case Conversion algorithm
toUppercase rather than toCasefold, which results in some subtle differences. For example,
Ω (U+2126 OHM SIGN) is mapped by toUppercase to itself but by toCasefold to
ω (U+03C9 GREEK SMALL LETTER OMEGA) along with Ω (U+03A9 GREEK CAPITAL
LETTER OMEGA), so "\u2126" is matched by /[ω]/ui and
/[\u03A9]/ui but not by /[ω]/i or /[\u03A9]/i. Also, no code
point outside the Basic Latin block is mapped to a code point within it, so strings such as
"\u017F ſ" and "\u212A K" are not matched by
/[a-z]/i.
22.2.2.8 Runtime Semantics: CompileCharacterClass
The syntax-directed
operation CompileCharacterClass takes argument rer (a RegExp
Record) and returns a
Record with
fields [[CharSet]] (a CharSet) and [[Invert]] (a Boolean). It is defined piecewise over the following productions:
ClassContents can expand into
a single ClassAtom and/or ranges of two
ClassAtom
separated by dashes. In the latter case the ClassContents includes all
characters between the first ClassAtom and the second ClassAtom,
inclusive; an error occurs if either ClassAtom does not represent a
single character (for example, if one is \w) or if the first ClassAtom's character value is
strictly greater than the second ClassAtom's character value.
Note 3
Even if the pattern ignores case, the case of the two ends of a range is significant in determining
which characters belong to the range. Thus, for example, the pattern /[E-F]/i matches
only the letters E, F, e, and f, while the pattern
/[E-f]/i matches all uppercase and lowercase letters in the Unicode Basic Latin block as
well as the symbols [, \, ], ^, _,
and `.
Note 4
A - character can be treated literally or it can denote a range. It is treated literally
if it is the first or last character of ClassContents, the beginning
or end limit of a range specification, or immediately follows a range specification.
2. Let c be the character whose character value is
cv.
3. Return the CharSet containing the single
character c.
Note 5
A ClassAtom
can use any of the
escape sequences that are allowed in the rest of the regular expression except for \b,
\B, and backreferences. Inside a CharacterClass,
\b means the backspace character, while \B and backreferences raise errors.
Using a backreference inside a ClassAtom causes an error.
4. Assert: p is a binary
Unicode property or binary property alias listed in the “Property name and
aliases” column of Table 68, or a
binary Unicode property of strings listed in the “Property name” column of
Table
69.
5. Let A be the CharSet
containing all CharSetElements whose character database definition includes the property p
with value “True”.
The result will often consist of two or more ranges. When UnicodeSets is true and
IgnoreCase is true, then MaybeSimpleCaseFolding(rer,
[Ā-č]) will include only the odd-numbered code points of that range.
1. Return the CharSet containing the single
character U+0008 (BACKSPACE).
22.2.2.9.1 CharacterRange ( A, B )
The abstract operation CharacterRange takes arguments A (a CharSet) and
B (a CharSet) and returns a CharSet. It performs the
following steps when called:
1. Assert: A and B
each contain exactly one character.
7. Return the CharSet containing all
characters with a character value in the inclusive interval from
i to j.
22.2.2.9.2 HasEitherUnicodeFlag ( rer )
The abstract operation HasEitherUnicodeFlag takes argument rer (a RegExp Record) and returns a
Boolean. It performs the following steps when called:
1. If rer.[[Unicode]] is
true or rer.[[UnicodeSets]] is
true, then
a. Return true.
2. Return false.
22.2.2.9.3 WordCharacters ( rer )
The abstract operation WordCharacters takes argument rer (a RegExp Record) and returns a
CharSet. Returns a CharSet containing the
characters considered "word characters" for the purposes of \b, \B,
\w, and \W It performs the following steps when called:
2. Let extraWordChars be the CharSet containing all
characters c such that c is not in basicWordChars but Canonicalize(rer,
c) is in basicWordChars.
3. Assert: extraWordChars is
empty unless HasEitherUnicodeFlag(rer)
is true and rer.[[IgnoreCase]] is
true.
4. Return the union of basicWordChars and
extraWordChars.
22.2.2.9.4 AllCharacters ( rer )
The abstract operation AllCharacters takes argument rer (a RegExp Record) and returns a
CharSet. Returns the set of “all
characters” according to the regular expression flags. It performs the following steps when called:
1. If rer.[[UnicodeSets]] is
true and rer.[[IgnoreCase]] is
true, then
a. Return the CharSet
containing all Unicode code points c that do not have a Simple Case Folding
mapping (that is, scf(c)=c).
a. Return the CharSet
containing all code point values.
3. Else,
a. Return the CharSet
containing all code unit values.
22.2.2.9.5 MaybeSimpleCaseFolding ( rer, A )
The abstract operation MaybeSimpleCaseFolding takes arguments rer (a RegExp Record) and
A (a CharSet) and returns a CharSet. If rer.[[UnicodeSets]] is false or rer.[[IgnoreCase]] is false, it returns A. Otherwise, it
uses the Simple Case Folding
(scf(cp)) definitions in the file
CaseFolding.txt of
the Unicode Character Database (each of which maps a single code point to another single code point) to
map each CharSetElement of
A character-by-character into a canonical form and returns the resulting CharSet. It performs the
following steps when called:
1. If rer.[[UnicodeSets]] is
false or rer.[[IgnoreCase]] is
false, return A.
The abstract operation CharacterComplement takes arguments rer (a RegExp Record) and
S (a CharSet) and returns a CharSet. It performs the
following steps when called:
2. Return the CharSet containing the
CharSetElements of A which are not also CharSetElements of S.
22.2.2.9.7 UnicodeMatchProperty ( rer, p )
The abstract operation UnicodeMatchProperty takes arguments rer (a RegExp Record) and
p (ECMAScript source text) and
returns a Unicode property name. It performs the
following steps when called:
1. If rer.[[UnicodeSets]] is
true and p is a Unicode property name
listed in the “Property name” column of Table
69, then
Implementations must support the Unicode property names and aliases listed in Table 67,
Table 68, and
Table
69. To ensure interoperability, implementations must not support any other property
names or aliases.
Note 1
For example, Script_Extensions (property name) and
scx (property alias) are valid, but script_extensions or Scx
aren't.
Note 2
The listed properties form a superset of what UTS18 RL1.2 requires.
Note 3
The spellings of entries in these tables (including casing) match the spellings used in the file PropertyAliases.txt
in the Unicode Character Database. The precise spellings in that file are guaranteed to be
stable.
Table 67: Non-binary Unicode property aliases and their canonical property names
The abstract operation UnicodeMatchPropertyValue takes arguments p (ECMAScript
source text) and
v (ECMAScript source text) and
returns a Unicode property value. It performs the following steps when called:
Implementations must support the Unicode property values and property value aliases listed in PropertyValueAliases.txt
for the properties listed in Table 67. To
ensure interoperability, implementations must not support any other property values or property value
aliases.
Note 1
For example, Xpeo and Old_Persian are valid
Script_Extensions values, but xpeo and Old Persian aren't.
The syntax-directed
operation CompileClassSetString takes argument rer (a RegExp
Record) and returns a
sequence of characters. It is defined piecewise over the following productions:
5. If F contains any code unit other than
"d", "g", "i", "m",
"s", "u", "v", or "y", or
if F contains any code unit more than once, throw a SyntaxError
exception.
6. If F contains "i", let i
be true; else let i be false.
7. If F contains "m", let m
be true; else let m be false.
8. If F contains "s", let s
be true; else let s be false.
9. If F contains "u", let u
be true; else let u be false.
10. If F contains "v", let
v be true; else let v be false.
a. Let patternText be the result of interpreting each
of P's 16-bit elements as a Unicode BMP code point. UTF-16 decoding is not applied to
the elements.
13. Let parseResult be ParsePattern(patternText,
u, v).
14. If parseResult is a non-empty List of
SyntaxError objects, throw a SyntaxError exception.
19. Let rer be the RegExp
Record { [[IgnoreCase]]: i, [[Multiline]]: m, [[DotAll]]: s,
[[Unicode]]: u, [[UnicodeSets]]:
v, [[CapturingGroupsCount]]:
capturingGroupsCount }.
20. Set obj.[[RegExpRecord]] to
rer.
21. Set obj.[[RegExpMatcher]] to
CompilePattern of
parseResult with argument rer.
22.2.3.4 Static Semantics: ParsePattern ( patternText,
u, v )
The abstract operation ParsePattern takes arguments patternText (a sequence of Unicode code
points), u (a Boolean), and v (a Boolean) and returns a Parse Node or a non-empty
List of
SyntaxError objects.
is the initial value of the "RegExp" property of the global
object.
creates and initializes a new RegExp object when called as a constructor.
when called as a function rather than as a constructor, returns either a new
RegExp object, or the argument itself if the only argument is a RegExp object.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit
the specified RegExp behaviour must include a super call to the RegExp constructor to create and initialize
subclass instances with the necessary internal slots.
22.2.4.1 RegExp ( pattern, flags )
This function performs the following steps when called:
If pattern is supplied using a StringLiteral, the usual
escape sequence substitutions are performed before the String is processed by this function. If
pattern must contain an escape sequence to be recognized by this function, any U+005C (REVERSE
SOLIDUS) code points must be escaped within the StringLiteral to prevent them
being removed when the contents of the StringLiteral are formed.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.
22.2.5.2 get RegExp [ @@species ]
RegExp[@@species] is an accessor property whose set
accessor function is undefined. Its get accessor function performs the following steps
when called:
1. Return the this value.
The value of the "name" property of this function is "get
[Symbol.species]".
Note
RegExp prototype methods normally use their this value's constructor to create a derived
object. However, a subclass constructor may over-ride that
default behaviour by redefining its @@species property.
The RegExp prototype object does not have a "valueOf" property of its own; however,
it inherits the "valueOf" property from the Object prototype
object.
22.2.6.1 RegExp.prototype.constructor
The initial value of RegExp.prototype.constructor is %RegExp%.
22.2.6.2 RegExp.prototype.exec ( string )
This method searches string for an occurrence of the regular expression pattern and returns an
Array containing the results of the match, or null if string did not match.
RegExp.prototype.dotAll is an accessor property whose set
accessor function is undefined. Its get accessor function performs the following steps
when called:
1. Let R be the this value.
2. Let cu be the code unit 0x0073 (LATIN SMALL LETTER S).
RegExp.prototype.flags is an accessor property whose set
accessor function is undefined. Its get accessor function performs the following steps
when called:
RegExp.prototype.global is an accessor property whose set
accessor function is undefined. Its get accessor function performs the following steps
when called:
1. Let R be the this value.
2. Let cu be the code unit 0x0067 (LATIN SMALL LETTER G).
RegExp.prototype.hasIndices is an accessor property whose set
accessor function is undefined. Its get accessor function performs the following steps
when called:
1. Let R be the this value.
2. Let cu be the code unit 0x0064 (LATIN SMALL LETTER D).
RegExp.prototype.ignoreCase is an accessor property whose set
accessor function is undefined. Its get accessor function performs the following steps
when called:
1. Let R be the this value.
2. Let cu be the code unit 0x0069 (LATIN SMALL LETTER I).
c. Perform ? Set(rx,
"lastIndex", 𝔽(nextIndex),
true).
4. Set n to n + 1.
The value of the "name" property of this method is
"[Symbol.match]".
Note
The @@match property is
used by the IsRegExp abstract operation to
identify objects that have the basic behaviour of regular expressions. The absence of a @@match property or the
existence of such a property whose value does not Boolean coerce to true indicates
that the object is not intended to be used as a regular expression object.
The value of the "name" property of this method is
"[Symbol.matchAll]".
22.2.6.10 get RegExp.prototype.multiline
RegExp.prototype.multiline is an accessor property whose set
accessor function is undefined. Its get accessor function performs the following steps
when called:
1. Let R be the this value.
2. Let cu be the code unit 0x006D (LATIN SMALL LETTER M).
iv. NOTE: When n = 1, the preceding step puts the
first element into captures (at index 0). More generally, the
nth capture (the characters captured by the nth
set of capturing parentheses) is at captures[n - 1].
1. Set namedCaptures to ? ToObject(namedCaptures).
ii. Let replacement be ? GetSubstitution(matched, S,
position, captures, namedCaptures, replaceValue).
m. If position ≥ nextSourcePosition, then
i. NOTE: position should not normally move
backwards. If it does, it is an indication of an ill-behaving RegExp subclass or use of an
access triggered side-effect to change the global flag or other characteristics of
rx. In such cases, the corresponding substitution is ignored.
ii. Set accumulatedResult to the string-concatenation
of accumulatedResult, the substring of S
from nextSourcePosition to position, and replacement.
iii. Set nextSourcePosition to
position + matchLength.
16. If nextSourcePosition ≥ lengthS, return
accumulatedResult.
The value of the "name" property of this method is
"[Symbol.search]".
Note
The "lastIndex" and "global" properties of this RegExp object
are ignored when performing the search. The "lastIndex" property is left unchanged.
22.2.6.13 get RegExp.prototype.source
RegExp.prototype.source is an accessor property whose set
accessor function is undefined. Its get accessor function performs the following steps
when called:
The abstract operation EscapeRegExpPattern takes arguments P (a String) and F (a
String) and returns a String. It performs the following steps when called:
1. If F contains "v", then
a. Let patternSymbol be Pattern[+UnicodeMode,
+UnicodeSetsMode].
2. Else if F contains "u", then
a. Let patternSymbol be Pattern[+UnicodeMode,
~UnicodeSetsMode].
3. Else,
a. Let patternSymbol be Pattern[~UnicodeMode,
~UnicodeSetsMode].
4. Let S be a String in the form of a
patternSymbol equivalent to P interpreted as UTF-16 encoded Unicode code
points (6.1.4),
in which certain code points are escaped as described below. S may or may not differ from
P; however, the Abstract Closure that
would result from evaluating S as a patternSymbol must behave identically to
the Abstract Closure given
by the constructed object's [[RegExpMatcher]] internal slot. Multiple calls
to this abstract operation using the same values for P and F must produce
identical results.
5. The code points / or any LineTerminator occurring
in the pattern shall be escaped in S as necessary to ensure that the string-concatenation of
"/", S, "/", and F can be parsed (in an
appropriate lexical context) as a RegularExpressionLiteral
that behaves identically to the constructed regular expression. For example, if P is
"/", then S could be "\/" or
"\u002F", among other possibilities, but not "/", because
/// followed by F would be parsed as a SingleLineComment
rather than a RegularExpressionLiteral.
If P is the empty String, this specification can be met by letting S be
"(?:)".
This method returns an Array into which substrings of the result of converting string to a
String have been stored. The substrings are determined by searching from left to right for matches of
the this value regular expression; these occurrences are not part of any String in
the returned array, but serve to divide up the String value.
The this value may be an empty regular expression or a regular expression that can
match an empty String. In this case, the regular expression does not match the empty
substring at the beginning or end of the input String, nor does it match
the empty substring at the end of the previous separator match. (For
example, if the regular expression matches the empty String, the String is split up into individual
code unit elements; the length of the result array equals the length of the String, and each
substring contains one code unit.) Only the first match at a given index of
the String is considered, even if backtracking could yield a non-empty
substring match at that index. (For example,
/a*?/[Symbol.split]("ab") evaluates to the array ["a", "b"], while
/a*/[Symbol.split]("ab") evaluates to the array ["","b"].)
If string is (or converts to) the empty String, the result depends on whether the regular
expression can match the empty String. If it can, the result array contains no elements. Otherwise,
the result array contains one element, which is the empty String.
If the regular expression contains capturing parentheses, then each time separator is
matched the results (including any undefined results) of the capturing parentheses
are spliced into the output array. For example,
The value of the "name" property of this method is
"[Symbol.split]".
Note 2
This method ignores the value of the "global" and "sticky"
properties of this RegExp object.
22.2.6.15 get RegExp.prototype.sticky
RegExp.prototype.sticky is an accessor property whose set
accessor function is undefined. Its get accessor function performs the following steps
when called:
1. Let R be the this value.
2. Let cu be the code unit 0x0079 (LATIN SMALL LETTER Y).
The returned String has the form of a RegularExpressionLiteral
that evaluates to another RegExp object with the same behaviour as this object.
22.2.6.18 get RegExp.prototype.unicode
RegExp.prototype.unicode is an accessor property whose set
accessor function is undefined. Its get accessor function performs the following steps
when called:
1. Let R be the this value.
2. Let cu be the code unit 0x0075 (LATIN SMALL LETTER U).
RegExp.prototype.unicodeSets is an accessor property whose set
accessor function is undefined. Its get accessor function performs the following steps
when called:
1. Let R be the this value.
2. Let cu be the code unit 0x0076 (LATIN SMALL LETTER V).
The abstract operation RegExpExec takes arguments R (an Object) and S (a String)
and returns either a normal completion
containing either an Object or null, or a throw
completion. It performs the following steps when called:
If a callable "exec" property is not found this algorithm falls back to attempting
to use the built-in RegExp matching algorithm. This provides compatible behaviour for code written for
prior editions where most built-in algorithms that use regular expressions did not perform a dynamic
property lookup of "exec".
22.2.7.2 RegExpBuiltinExec ( R, S )
The abstract operation RegExpBuiltinExec takes arguments R (an initialized RegExp instance)
and S (a String) and returns either a normal completion
containing either an Array exotic object or
null, or a throw
completion. It performs the following steps when called:
1. Let length be the length of S.
2. Let lastIndex be ℝ(? ToLength(? Get(R,
"lastIndex"))).
3. Let flags be R.[[OriginalFlags]].
4. If flags contains "g", let
global be true; else let global be false.
5. If flags contains "y", let
sticky be true; else let sticky be false.
6. If flags contains "d", let
hasIndices be true; else let hasIndices be
false.
7. If global is false and
sticky is false, set lastIndex to 0.
8. Let matcher be R.[[RegExpMatcher]].
9. If flags contains "u" or
flags contains "v", let fullUnicode be
true; else let fullUnicode be false.
10. Let matchSucceeded be false.
11. If fullUnicode is true, let
input be StringToCodePoints(S).
Otherwise, let input be a List
whose elements are the code units that are the elements of S.
12. NOTE: Each element of input is considered to be a
character.
The abstract operation AdvanceStringIndex takes arguments S (a String), index (a
non-negative integer), and unicode (a
Boolean) and returns an integer. It performs the following steps
when called:
The abstract operation GetStringIndex takes arguments S (a String) and
codePointIndex (a non-negative integer) and returns a non-negative
integer. It interprets S as a
sequence of UTF-16 encoded code points, as described in 6.1.4,
and returns the code unit index corresponding to code point index codePointIndex when such an
index exists. Otherwise, it returns the length of S. It performs the following steps when
called:
1. If S is the empty String, return 0.
2. Let len be the length of S.
3. Let codeUnitCount be 0.
4. Let codePointCount be 0.
5. Repeat, while codeUnitCount < len,
a. If codePointCount = codePointIndex,
return codeUnitCount.
The number of code units from the start of a string at which the match ends (exclusive).
22.2.7.6 GetMatchString ( S, match )
The abstract operation GetMatchString takes arguments S (a String) and match (a
Match Record) and returns a
String. It performs the following steps when called:
1. Assert: match.[[StartIndex]] ≤ match.[[EndIndex]] ≤ the
length of S.
2. Return the substring of S from
match.[[StartIndex]] to match.[[EndIndex]].
22.2.7.7 GetMatchIndexPair ( S, match )
The abstract operation GetMatchIndexPair takes arguments S (a String) and match (a
Match Record) and returns an
Array. It performs the following steps when called:
1. Assert: match.[[StartIndex]] ≤ match.[[EndIndex]] ≤ the
length of S.
The abstract operation MakeMatchIndicesIndexPairArray takes arguments S (a String),
indices (a List of
either Match Records or
undefined), groupNames (a List of
either Strings or undefined), and hasGroups (a Boolean) and returns an
Array. It performs the following steps when called:
RegExp instances are ordinary objects that inherit
properties from the RegExp prototype
object. RegExp instances have internal slots [[OriginalSource]],
[[OriginalFlags]], [[RegExpRecord]], and [[RegExpMatcher]]. The value of the [[RegExpMatcher]]
internal slot is an Abstract Closure representation
of the Pattern of
the RegExp object.
Note
Prior to ECMAScript 2015, RegExp instances were specified as having the own data
properties"source", "global",
"ignoreCase", and "multiline". Those properties are now specified
as accessor properties of
RegExp.prototype.
RegExp instances also have the following property:
22.2.8.1 lastIndex
The value of the "lastIndex" property specifies the String index at which to start the
next match. It is coerced to an integral Number when used (see
22.2.7.2). This property
shall have the attributes { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]:
false }.
22.2.9 RegExp String Iterator Objects
A RegExp String Iterator is an object, that represents a specific iteration over some specific String
instance object, matching against some specific RegExp instance object. There is not a named constructor for RegExp String Iterator
objects. Instead, RegExp String Iterator objects are created by calling certain methods of RegExp instance
objects.
The abstract operation CreateRegExpStringIterator takes arguments R (an Object), S
(a String), global (a Boolean), and fullUnicode (a Boolean) and returns a Generator.
It performs the following steps when called:
1. Let closure be a new Abstract Closure with no
parameters that captures R, S, global, and fullUnicode and
performs the following steps when called:
is the initial value of the "Array" property of the global
object.
creates and initializes a new Array when called as a constructor.
also creates and initializes a new Array when called as a function rather than as a constructor. Thus the function call
Array(…) is equivalent to the object creation expression new Array(…) with the
same arguments.
is a function whose behaviour differs based upon the number and types of its arguments.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit
the exotic Array behaviour must include a super call to the Array constructor to initialize subclass
instances that are Array exotic objects. However,
most of the Array.prototype methods are generic methods that are not dependent upon their
this value being an Array exotic object.
23.1.1.1 Array ( ...values )
This function performs the following steps when called:
1. If NewTarget is undefined, let
newTarget be the active function object;
else let newTarget be NewTarget.
This method is an intentionally generic factory method; it does not require that its
this value be the Array constructor. Therefore it can be
transferred to or inherited by any other constructors that may be called
with a single numeric argument.
23.1.2.2 Array.isArray ( arg )
This function performs the following steps when called:
This method is an intentionally generic factory method; it does not require that its
this value be the Array constructor. Therefore it can be
transferred to or inherited by other constructors that may be called
with a single numeric argument.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.
23.1.2.5 get Array [ @@species ]
Array[@@species] is an accessor property whose set
accessor function is undefined. Its get accessor function performs the following steps
when called:
1. Return the this value.
The value of the "name" property of this function is "get
[Symbol.species]".
Note
Array prototype methods normally use their this value's constructor to create a derived
object. However, a subclass constructor may over-ride that
default behaviour by redefining its @@species property.
23.1.3 Properties of the Array Prototype Object
The Array prototype object:
is %Array.prototype%.
is an Array exotic object and has
the internal methods specified for such objects.
has a "length" property whose initial value is +0𝔽 and
whose attributes are { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]:
false }.
The Array prototype object is specified to be an Array exotic
object to ensure compatibility with ECMAScript code that was created prior to the
ECMAScript 2015 specification.
The explicit setting of the "length" property in step 6 is
intended to ensure the length is correct when the final non-empty element of items has
trailing holes or when A is not a built-in Array.
Note 2
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a method.
The initial value of Array.prototype.constructor is %Array%.
23.1.3.4 Array.prototype.copyWithin ( target, start [
, end ] )
Note 1
The end argument is optional. If it is not provided, the length of the
this value is used.
Note 2
If target is negative, it is treated as length +
target where length is the length of the array. If start
is negative, it is treated as length + start.
If end is negative, it is treated as length +
end.
This method performs the following steps when called:
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a method.
23.1.3.5 Array.prototype.entries ( )
This method performs the following steps when called:
callbackfn should be a function that accepts three arguments and returns a value that is
coercible to a Boolean value. every calls callbackfn once for each element
present in the array, in ascending order, until it finds one where callbackfn returns
false. If such an element is found, every immediately returns
false. Otherwise, if callbackfn returned true for all
elements, every will return true. callbackfn is called only
for elements of the array which actually exist; it is not called for missing elements of the array.
If a thisArg parameter is provided, it will be used as the this value
for each invocation of callbackfn. If it is not provided, undefined is
used instead.
callbackfn is called with three arguments: the value of the element, the index of the
element, and the object being traversed.
every does not directly mutate the object on which it is called but the object may be
mutated by the calls to callbackfn.
The range of elements processed by every is set before the first call to
callbackfn. Elements which are appended to the array after the call to every
begins will not be visited by callbackfn. If existing elements of the array are changed,
their value as passed to callbackfn will be the value at the time every visits
them; elements that are deleted after the call to every begins and before being visited
are not visited. every acts like the "for all" quantifier in mathematics. In particular,
for an empty array, it returns true.
This method performs the following steps when called:
ii. Let testResult be ToBoolean(?
Call(callbackfn, thisArg, «
kValue, 𝔽(k),
O »)).
iii. If testResult is false,
return false.
d. Set k to k + 1.
6. Return true.
Note 2
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a method.
23.1.3.7 Array.prototype.fill ( value [ , start [ ,
end ] ] )
Note 1
The start argument is optional. If it is not provided, +0𝔽
is used.
The end argument is optional. If it is not provided, the length of the
this value is used.
Note 2
If start is negative, it is treated as length +
start where length is the length of the array. If end is
negative, it is treated as length + end.
This method performs the following steps when called:
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a method.
callbackfn should be a function that accepts three arguments and returns a value that is
coercible to a Boolean value. filter calls callbackfn once for each element in
the array, in ascending order, and constructs a new array of all the values for which
callbackfn returns true. callbackfn is called only for
elements of the array which actually exist; it is not called for missing elements of the array.
If a thisArg parameter is provided, it will be used as the this value
for each invocation of callbackfn. If it is not provided, undefined is
used instead.
callbackfn is called with three arguments: the value of the element, the index of the
element, and the object being traversed.
filter does not directly mutate the object on which it is called but the object may be
mutated by the calls to callbackfn.
The range of elements processed by filter is set before the first call to
callbackfn. Elements which are appended to the array after the call to filter
begins will not be visited by callbackfn. If existing elements of the array are changed
their value as passed to callbackfn will be the value at the time filter
visits them; elements that are deleted after the call to filter begins and before being
visited are not visited.
This method performs the following steps when called:
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a method.
This method calls predicate once for each element of the array, in ascending index order,
until it finds one where predicate returns a value that coerces to true.
If such an element is found, find immediately returns that element value. Otherwise,
find returns undefined.
3. Let findRec be ? FindViaPredicate(O, len,
ascending, predicate, thisArg).
4. Return findRec.[[Value]].
Note 2
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a method.
This method calls predicate once for each element of the array, in ascending index order,
until it finds one where predicate returns a value that coerces to true.
If such an element is found, findIndex immediately returns the index of that element
value. Otherwise, findIndex returns -1.
3. Let findRec be ? FindViaPredicate(O, len,
ascending, predicate, thisArg).
4. Return findRec.[[Index]].
Note 2
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a method.
This method calls predicate once for each element of the array, in descending index order,
until it finds one where predicate returns a value that coerces to true.
If such an element is found, findLast immediately returns that element value. Otherwise,
findLast returns undefined.
3. Let findRec be ? FindViaPredicate(O, len,
descending, predicate, thisArg).
4. Return findRec.[[Value]].
Note 2
This method is intentionally generic; it does not require that its this value be
an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
This method calls predicate once for each element of the array, in descending index order,
until it finds one where predicate returns a value that coerces to true.
If such an element is found, findLastIndex immediately returns the index of that element
value. Otherwise, findLastIndex returns -1.
3. Let findRec be ? FindViaPredicate(O, len,
descending, predicate, thisArg).
4. Return findRec.[[Index]].
Note 2
This method is intentionally generic; it does not require that its this value be
an Array object. Therefore it can be transferred to other kinds of objects for use as a method.
23.1.3.12.1 FindViaPredicate ( O, len,
direction, predicate, thisArg )
O should be an array-like object or a
TypedArray. This operation calls
predicate once for each element of O, in either ascending index order or
descending index order (as indicated by direction), until it finds one where
predicate returns a value that coerces to true. At that point, this
operation returns a Record
that gives the index and value of the element found. If no such element is found, this operation returns
a Record
that specifies -1𝔽 for the index and undefined for the
value.
predicate should be a function. When called for an element of the array, it is passed three
arguments: the value of the element, the index of the element, and the object being traversed. Its
return value will be coerced to a Boolean value.
thisArg will be used as the this value for each invocation of
predicate.
This operation does not directly mutate the object on which it is called, but the object may be mutated
by the calls to predicate.
The range of elements processed is set before the first call to predicate, just before the
traversal begins. Elements that are appended to the array after this will not be visited by
predicate. If existing elements of the array are changed, their value as passed to
predicate will be the value at the time that this operation visits them. Elements that are
deleted after traversal begins and before being visited are still visited and are either looked up from
the prototype or are undefined.
It performs the following steps when called:
1. If IsCallable(predicate)
is false, throw a TypeError exception.
2. If direction is ascending,
then
a. Let indices be a List
of the integers in the interval from 0 (inclusive) to
len (exclusive), in ascending order.
3. Else,
a. Let indices be a List
of the integers in the interval from 0 (inclusive) to
len (exclusive), in descending order.
callbackfn should be a function that accepts three arguments. forEach calls
callbackfn once for each element present in the array, in ascending order.
callbackfn is called only for elements of the array which actually exist; it is not called
for missing elements of the array.
If a thisArg parameter is provided, it will be used as the this value
for each invocation of callbackfn. If it is not provided, undefined is
used instead.
callbackfn is called with three arguments: the value of the element, the index of the
element, and the object being traversed.
forEach does not directly mutate the object on which it is called but the object may be
mutated by the calls to callbackfn.
The range of elements processed by forEach is set before the first call to
callbackfn. Elements which are appended to the array after the call to forEach
begins will not be visited by callbackfn. If existing elements of the array are changed,
their value as passed to callbackfn will be the value at the time forEach
visits them; elements that are deleted after the call to forEach begins and before being
visited are not visited.
This method performs the following steps when called:
ii. Perform ? Call(callbackfn, thisArg, «
kValue, 𝔽(k),
O »).
d. Set k to k + 1.
6. Return undefined.
Note 2
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a method.
This method compares searchElement to the elements of the array, in ascending order, using
the SameValueZero algorithm, and
if found at any position, returns true; otherwise, it returns
false.
The optional second argument fromIndex defaults to +0𝔽
(i.e. the whole array is searched). If it is greater than or equal to the length of the array,
false is returned, i.e. the array will not be searched. If it is less than
-0𝔽, it is used as the offset from the end of the array to compute
fromIndex. If the computed index is less than or equal to
+0𝔽, the whole array will be searched.
This method performs the following steps when called:
b. If SameValueZero(searchElement,
elementK) is true, return true.
c. Set k to k + 1.
11. Return false.
Note 2
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a method.
Note 3
This method intentionally differs from the similar indexOf method in two ways. First, it
uses the SameValueZero algorithm,
instead of IsStrictlyEqual, allowing
it to detect NaN array elements. Second, it does not skip missing array elements,
instead treating them as undefined.
This method compares searchElement to the elements of the array, in ascending order, using the
IsStrictlyEqual algorithm, and
if found at one or more indices, returns the smallest such index; otherwise, it returns
-1𝔽.
Note 1
The optional second argument fromIndex defaults to +0𝔽
(i.e. the whole array is searched). If it is greater than or equal to the length of the array,
-1𝔽 is returned, i.e. the array will not be searched. If it is less
than -0𝔽, it is used as the offset from the end of the array to compute
fromIndex. If the computed index is less than or equal to
+0𝔽, the whole array will be searched.
This method performs the following steps when called:
ii. If IsStrictlyEqual(searchElement,
elementK) is true, return 𝔽(k).
c. Set k to k + 1.
11. Return -1𝔽.
Note 2
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a method.
23.1.3.18 Array.prototype.join ( separator )
This method converts the elements of the array to Strings, and then concatenates these Strings, separated
by occurrences of the separator. If no separator is provided, a single comma is used as the
separator.
This method is intentionally generic; it does not require that its this value be
an Array. Therefore, it can be transferred to other kinds of objects for use as a method.
23.1.3.19 Array.prototype.keys ( )
This method performs the following steps when called:
This method compares searchElement to the elements of the array in descending order using
the IsStrictlyEqual algorithm,
and if found at one or more indices, returns the largest such index; otherwise, it returns
-1𝔽.
The optional second argument fromIndex defaults to the array's length minus one (i.e. the
whole array is searched). If it is greater than or equal to the length of the array, the whole array
will be searched. If it is less than -0𝔽, it is used as the offset from
the end of the array to compute fromIndex. If the computed index is less than or equal to
+0𝔽, -1𝔽 is returned.
This method performs the following steps when called:
ii. If IsStrictlyEqual(searchElement,
elementK) is true, return 𝔽(k).
c. Set k to k - 1.
9. Return -1𝔽.
Note 2
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a method.
callbackfn should be a function that accepts three arguments. map calls
callbackfn once for each element in the array, in ascending order, and constructs a new
Array from the results. callbackfn is called only for elements of the array which actually
exist; it is not called for missing elements of the array.
If a thisArg parameter is provided, it will be used as the this value
for each invocation of callbackfn. If it is not provided, undefined is
used instead.
callbackfn is called with three arguments: the value of the element, the index of the
element, and the object being traversed.
map does not directly mutate the object on which it is called but the object may be
mutated by the calls to callbackfn.
The range of elements processed by map is set before the first call to
callbackfn. Elements which are appended to the array after the call to map
begins will not be visited by callbackfn. If existing elements of the array are changed,
their value as passed to callbackfn will be the value at the time map visits
them; elements that are deleted after the call to map begins and before being visited are
not visited.
This method performs the following steps when called:
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a method.
23.1.3.22 Array.prototype.pop ( )
Note 1
This method removes the last element of the array and returns it.
This method performs the following steps when called:
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a method.
23.1.3.23 Array.prototype.push ( ...items )
Note 1
This method appends the arguments to the end of the array, in the order in which they appear. It
returns the new length of the array.
This method performs the following steps when called:
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a method.
callbackfn should be a function that takes four arguments. reduce calls the
callback, as a function, once for each element after the first element present in the array, in
ascending order.
callbackfn is called with four arguments: the previousValue (value from the
previous call to callbackfn), the currentValue (value of the current element),
the currentIndex, and the object being traversed. The first time that callback is called,
the previousValue and currentValue can be one of two values. If an
initialValue was supplied in the call to reduce, then previousValue
will be initialValue and currentValue will be the first value in the array. If
no initialValue was supplied, then previousValue will be the first value in the
array and currentValue will be the second. It is a TypeError if the
array contains no elements and initialValue is not provided.
reduce does not directly mutate the object on which it is called but the object may be
mutated by the calls to callbackfn.
The range of elements processed by reduce is set before the first call to
callbackfn. Elements that are appended to the array after the call to reduce
begins will not be visited by callbackfn. If existing elements of the array are changed,
their value as passed to callbackfn will be the value at the time reduce
visits them; elements that are deleted after the call to reduce begins and before being
visited are not visited.
This method performs the following steps when called:
ii. Set accumulator to ? Call(callbackfn,
undefined, « accumulator, kValue, 𝔽(k),
O »).
d. Set k to k + 1.
10. Return accumulator.
Note 2
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a method.
callbackfn should be a function that takes four arguments. reduceRight calls
the callback, as a function, once for each element after the first element present in the array, in
descending order.
callbackfn is called with four arguments: the previousValue (value from the
previous call to callbackfn), the currentValue (value of the current element),
the currentIndex, and the object being traversed. The first time the function is called,
the previousValue and currentValue can be one of two values. If an
initialValue was supplied in the call to reduceRight, then
previousValue will be initialValue and currentValue will be the last
value in the array. If no initialValue was supplied, then previousValue will be
the last value in the array and currentValue will be the second-to-last value. It is a
TypeError if the array contains no elements and initialValue is not
provided.
reduceRight does not directly mutate the object on which it is called but the object may
be mutated by the calls to callbackfn.
The range of elements processed by reduceRight is set before the first call to
callbackfn. Elements that are appended to the array after the call to
reduceRight begins will not be visited by callbackfn. If existing elements of
the array are changed by callbackfn, their value as passed to callbackfn will be
the value at the time reduceRight visits them; elements that are deleted after the call
to reduceRight begins and before being visited are not visited.
This method performs the following steps when called:
ii. Set accumulator to ? Call(callbackfn,
undefined, « accumulator, kValue, 𝔽(k),
O »).
d. Set k to k - 1.
10. Return accumulator.
Note 2
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a method.
23.1.3.26 Array.prototype.reverse ( )
Note 1
This method rearranges the elements of the array so as to reverse their order. It returns the object
as the result of the call.
This method performs the following steps when called:
i. Assert: lowerExists
and upperExists are both false.
ii. NOTE: No action is required.
l. Set lower to lower + 1.
6. Return O.
Note 2
This method is intentionally generic; it does not require that its this value be
an Array. Therefore, it can be transferred to other kinds of objects for use as a method.
23.1.3.27 Array.prototype.shift ( )
This method removes the first element of the array and returns it.
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a method.
23.1.3.28 Array.prototype.slice ( start, end )
This method returns an array containing the elements of the array from element start up to,
but not including, element end (or through the end of the array if end is
undefined). If start is negative, it is treated as length + start where length is the length of
the array. If end is negative, it is treated as length +
end where length is the length of the array.
The explicit setting of the "length" property in step 15 is
intended to ensure the length is correct even when A is not a built-in Array.
Note 2
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a method.
callbackfn should be a function that accepts three arguments and returns a value that is
coercible to a Boolean value. some calls callbackfn once for each element
present in the array, in ascending order, until it finds one where callbackfn returns
true. If such an element is found, some immediately returns
true. Otherwise, some returns false.
callbackfn is called only for elements of the array which actually exist; it is not called
for missing elements of the array.
If a thisArg parameter is provided, it will be used as the this value
for each invocation of callbackfn. If it is not provided, undefined is
used instead.
callbackfn is called with three arguments: the value of the element, the index of the
element, and the object being traversed.
some does not directly mutate the object on which it is called but the object may be
mutated by the calls to callbackfn.
The range of elements processed by some is set before the first call to
callbackfn. Elements that are appended to the array after the call to some
begins will not be visited by callbackfn. If existing elements of the array are changed,
their value as passed to callbackfn will be the value at the time that some
visits them; elements that are deleted after the call to some begins and before being
visited are not visited. some acts like the "exists" quantifier in mathematics. In
particular, for an empty array, it returns false.
This method performs the following steps when called:
ii. Let testResult be ToBoolean(?
Call(callbackfn, thisArg, «
kValue, 𝔽(k),
O »)).
iii. If testResult is true,
return true.
d. Set k to k + 1.
6. Return false.
Note 2
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a method.
23.1.3.30 Array.prototype.sort ( comparefn )
This method sorts the elements of this array. The sort must be stable (that is, elements that compare
equal must remain in their original order). If comparefn is not undefined,
it should be a function that accepts two arguments x and y and returns a negative
Number if x < y, a positive Number if x > y, or a zero
otherwise.
It performs the following steps when called:
1. If comparefn is not
undefined and IsCallable(comparefn)
is false, throw a TypeError exception.
6. Let itemCount be the number of elements in
sortedList.
7. Let j be 0.
8. Repeat, while j < itemCount,
a. Perform ? Set(obj, ! ToString(𝔽(j)),
sortedList[j], true).
b. Set j to j + 1.
9. NOTE: The call to SortIndexedProperties
in step 5 uses
skip-holes. The remaining indices are deleted to preserve the number of holes
that were detected and excluded from the sort.
Because non-existent property values always compare greater than undefined
property values, and undefined always compares greater than any other value (see
CompareArrayElements),
undefined property values always sort to the end of the result, followed by
non-existent property values.
This method is intentionally generic; it does not require that its this value be
an Array. Therefore, it can be transferred to other kinds of objects for use as a method.
The sort order is the ordering of items after
completion of step 4 of the algorithm above. The
sort
order is implementation-defined if
SortCompare is not a consistent comparator for
the elements of items. When SortIndexedProperties is invoked by Array.prototype.sort,
the sort
order is also implementation-defined if
comparefn is undefined, and all applications of ToString, to any
specific value passed as an argument to SortCompare, do not produce the same result.
There must be some mathematical permutation π of the non-negative integers less than
itemCount, such that for every non-negative integerj less than
itemCount, the element old[j] is exactly the
same as new[π(j)].
Then for all non-negative integersj and
k, each less than itemCount, if ℝ(SortCompare(old[j],
old[k])) < 0, then π(j) <
π(k).
Here the notation old[j] is used to refer to items[j] before step 4 is executed,
and the notation new[j] to refer to items[j] after step 4 has been
executed.
An abstract closure or function comparator is a consistent comparator for a set of values S if all of the requirements
below are met for all values a, b, and c (possibly the same value) in
the set S: The notation a <Cb means ℝ(comparator(a,
b)) < 0; a =Cb means ℝ(comparator(a,
b)) = 0; and a >Cb means ℝ(comparator(a,
b)) > 0.
Calling comparator(a, b) always returns the same value v
when given a specific pair of values a and b as its two arguments. Furthermore,
vis a
Number, and v is not NaN. Note that this implies that
exactly one of a <Cb, a =Cb,
and a >Cb will be true for a given pair of a and
b.
Calling comparator(a, b) does not modify obj or any object
on obj's prototype chain.
a =Ca (reflexivity)
If a =Cb, then b =Ca (symmetry)
If a =Cb and b =Cc, then
a =Cc (transitivity of =C)
If a <Cb and b <Cc, then
a <Cc (transitivity of <C)
If a >Cb and b >Cc, then
a >Cc (transitivity of >C)
Note
The above conditions are necessary and sufficient to ensure that comparator divides the
set S into equivalence classes and that these equivalence classes are totally ordered.
23.1.3.30.2 CompareArrayElements ( x, y,
comparefn )
This method deletes the deleteCount elements of the array starting at integer
indexstart and replaces them with the elements of items. It
returns an Array containing the deleted elements (if any).
This method performs the following steps when called:
The explicit setting of the "length" property in steps 15 and
20 is
intended to ensure the lengths are correct even when the objects are not built-in Arrays.
Note 3
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a method.
An ECMAScript implementation that includes the ECMA-402 Internationalization API must implement this
method as specified in the ECMA-402 specification. If an ECMAScript implementation does not include the
ECMA-402 API the following specification of this method is used.
Note 1
The first edition of ECMA-402 did not include a replacement specification for this method.
The meanings of the optional parameters to this method are defined in the ECMA-402 specification;
implementations that do not include ECMA-402 support must not use those parameter positions for anything
else.
This method performs the following steps when called:
This method converts the elements of the array to Strings using their toLocaleString
methods, and then concatenates these Strings, separated by occurrences of an implementation-defined
locale-sensitive separator String. This method is analogous to toString except that it is
intended to yield a locale-sensitive result corresponding with conventions of the host
environment's current
locale.
Note 3
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a method.
23.1.3.33 Array.prototype.toReversed ( )
This method performs the following steps when called:
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a method.
23.1.3.37 Array.prototype.unshift ( ...items )
This method prepends the arguments to the start of the array, such that their order within the array is
the same as the order in which they appear in the argument list.
This method is intentionally generic; it does not require that its this value be
an Array. Therefore it can be transferred to other kinds of objects for use as a method.
23.1.3.38 Array.prototype.values ( )
This method performs the following steps when called:
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
true }.
Note
The own property names of this object are property names that were not included as standard
properties of Array.prototype prior to the ECMAScript 2015 specification. These names are
ignored for with statement binding purposes in order to preserve the behaviour of
existing code that might use one of these names as a binding in an outer scope that is shadowed by a
with statement whose binding object is an Array.
The reason that "with" is not included in the unscopableList is because
it is already a reserved word.
Array instances have a "length" property, and a set of enumerable properties with
array
index names.
23.1.4.1 length
The "length" property of an Array instance is a data property
whose value is always numerically greater than the name of every configurable own property whose name is
an array
index.
The "length" property initially has the attributes { [[Writable]]: true, [[Enumerable]]:
false, [[Configurable]]: false }.
Note
Reducing the value of the "length" property has the side-effect of deleting own
array elements whose array index is between the old and
new length values. However, non-configurable properties can not be deleted. Attempting to set the
"length" property of an Array to a value that is numerically less than or equal to
the largest numeric own property name of an existing
non-configurable array-indexed property of the
array will result in the length being set to a numeric value that is one greater than that
non-configurable numeric own property name. See 10.4.2.1.
23.1.5 Array Iterator Objects
An Array Iterator is an object, that represents a specific iteration over some specific Array instance
object. There is not a named constructor for Array Iterator objects.
Instead, Array iterator objects are created by calling certain methods of Array instance objects.
23.1.5.1 CreateArrayIterator ( array, kind )
The abstract operation CreateArrayIterator takes arguments array (an Object) and
kind (key+value, key, or
value) and returns a Generator. It is used to create iterator objects for Array
methods that return such iterators. It performs the following steps when called:
1. Let closure be a new Abstract Closure with no
parameters that captures kind and array and performs the following steps when
called:
a. Let index be 0.
b. Repeat,
i. If array has a [[TypedArrayName]] internal slot, then
The initial value of the @@toStringTag property is
the String value "Array Iterator".
This property has the attributes { [[Writable]]: false,
[[Enumerable]]: false, [[Configurable]]: true }.
23.2 TypedArray Objects
A TypedArray presents an array-like view of an underlying binary data buffer (25.1). A TypedArray element type is the underlying binary
scalar data type that all elements of a TypedArray instance have. There is a distinct
TypedArrayconstructor, listed in Table 71, for each
of the supported element types. Each constructor in Table 71 has a
corresponding distinct prototype object.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.
23.2.2.4 get %TypedArray% [ @@species ]
%TypedArray%[@@species]
is an accessor property whose set
accessor function is undefined. Its get accessor function performs the following steps
when called:
1. Return the this value.
The value of the "name" property of this function is "get
[Symbol.species]".
%TypedArray%.prototype.buffer
is an accessor property whose set
accessor function is undefined. Its get accessor function performs the following steps
when called:
3. Assert: O has a [[ViewedArrayBuffer]] internal slot.
4. Let buffer be O.[[ViewedArrayBuffer]].
5. Return buffer.
23.2.3.3 get %TypedArray%.prototype.byteLength
%TypedArray%.prototype.byteLength
is an accessor property whose set
accessor function is undefined. Its get accessor function performs the following steps
when called:
%TypedArray%.prototype.byteOffset
is an accessor property whose set
accessor function is undefined. Its get accessor function performs the following steps
when called:
ii. If IsStrictlyEqual(searchElement,
elementK) is true, return 𝔽(k).
c. Set k to k - 1.
10. Return -1𝔽.
This method is not generic. The this value must be an object with a [[TypedArrayName]] internal slot.
23.2.3.21 get %TypedArray%.prototype.length
%TypedArray%.prototype.length
is an accessor property whose set
accessor function is undefined. Its get accessor function performs the following steps
when called:
This method sets multiple values in this TypedArray, reading the values from
source. The details differ based upon the type of source. The optional
offset value indicates the first element index in this TypedArray where values are
written. If omitted, it is assumed to be 0.
The abstract operation SetTypedArrayFromTypedArray takes arguments target (a TypedArray), targetOffset
(a non-negative integer or +∞), and source (a
TypedArray) and returns either a
normal completion
containingunused or a throw
completion. It sets multiple values in target, starting at index
targetOffset, reading the values from source. It performs the following steps when
called:
1. Let targetBuffer be target.[[ViewedArrayBuffer]].
15. If targetOffset = +∞, throw a
RangeError exception.
16. If srcLength + targetOffset >
targetLength, throw a RangeError exception.
17. If target.[[ContentType]]
is not source.[[ContentType]], throw a
TypeError exception.
18. If IsSharedArrayBuffer(srcBuffer)
is true, IsSharedArrayBuffer(targetBuffer)
is true, and srcBuffer.[[ArrayBufferData]] is
targetBuffer.[[ArrayBufferData]], let
sameSharedArrayBuffer be true; otherwise, let
sameSharedArrayBuffer be false.
19. If SameValue(srcBuffer,
targetBuffer) is true or sameSharedArrayBuffer is
true, then
The abstract operation SetTypedArrayFromArrayLike takes arguments target (a TypedArray), targetOffset
(a non-negative integer or +∞), and source
(an ECMAScript language
value, but not a TypedArray) and returns either a
normal completion
containingunused or a throw
completion. It sets multiple values in target, starting at index
targetOffset, reading the values from source. It performs the following steps when
called:
This is a distinct method that, except as described below, implements the same requirements as those of
Array.prototype.sort as defined in 23.1.3.30.
The implementation of this method may be optimized with the knowledge that the this
value is an object that has a fixed length and whose integer-indexed properties are not
sparse.
This method is not generic. The this value must be an object with a [[TypedArrayName]] internal slot.
It performs the following steps when called:
1. If comparefn is not undefined and
IsCallable(comparefn)
is false, throw a TypeError exception.
7. Let sortedList be ? SortIndexedProperties(obj, len,
SortCompare, read-through-holes).
8. Let j be 0.
9. Repeat, while j < len,
a. Perform ! Set(obj,
! ToString(𝔽(j)),
sortedList[j], true).
b. Set j to j + 1.
10. Return obj.
Note
Because NaN always compares greater than any other value (see CompareTypedArrayElements),
NaN property values always sort to the end of the result when comparefn
is not provided.
23.2.3.30 %TypedArray%.prototype.subarray ( start, end
)
This method returns a new TypedArray whose element type is the element type of this
TypedArray and whose ArrayBuffer is the ArrayBuffer of this TypedArray, referencing
the elements in the interval from start
(inclusive) to end (exclusive). If either start or end is negative, it
refers to an index from the end of the array, as opposed to from the beginning.
This is a distinct method that implements the same algorithm as
Array.prototype.toLocaleString as defined in 23.1.3.32
except that TypedArrayLength is called in
place of performing a [[Get]] of "length". The implementation
of the algorithm may be optimized with the knowledge that the this value has a fixed
length when the underlying buffer is not resizable and whose integer-indexed
properties are not sparse. However, such optimization must not introduce any observable changes in the
specified behaviour of the algorithm.
This method is not generic. ValidateTypedArray is
called with the this value and seq-cst as arguments prior to
evaluating the algorithm. If its result is an abrupt
completion that exception is thrown instead of evaluating the algorithm.
Note
If the ECMAScript implementation includes the ECMA-402 Internationalization API this method is based
upon the algorithm for Array.prototype.toLocaleString that is in the ECMA-402
specification.
23.2.3.32 %TypedArray%.prototype.toReversed ( )
This method performs the following steps when called:
The initial value of the @@iterator property is
%TypedArray.prototype.values%, defined in 23.2.3.35.
23.2.3.38 get %TypedArray%.prototype [ @@toStringTag ]
%TypedArray%.prototype[@@toStringTag]
is an accessor property whose set
accessor function is undefined. Its get accessor function performs the following steps
when called:
The abstract operation TypedArrayElementSize takes argument O (a TypedArray) and
returns a non-negative integer. It performs the following steps
when called:
1. Return the Element Size value specified in Table 71 for
O.[[TypedArrayName]].
23.2.4.6 TypedArrayElementType ( O )
The abstract operation TypedArrayElementType takes argument O (a TypedArray) and
returns a TypedArray element type. It
performs the following steps when called:
1. Return the Element Type value specified in Table 71 for
O.[[TypedArrayName]].
23.2.4.7 CompareTypedArrayElements ( x, y,
comparefn )
The abstract operation CompareTypedArrayElements takes arguments x (a Number or a BigInt),
y (a Number or a BigInt), and comparefn (a function object
or undefined) and returns either a normal completion
containing a Number or an abrupt
completion. It performs the following steps when called:
is an intrinsic object that has the structure described below, differing only in the name used as the
constructor name instead of
TypedArray, in Table 71.
is a function whose behaviour differs based upon the number and types of its arguments. The actual
behaviour of a call of TypedArray depends upon the number and kind of arguments that are passed
to it.
is not intended to be called as a function and will throw an exception when called in that manner.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit
the specified TypedArray behaviour must include a super call to the
TypedArrayconstructor to create and initialize
the subclass instance with the internal state necessary to support the %TypedArray%.prototype
built-in methods.
23.2.5.1TypedArray ( ...args )
Each TypedArrayconstructor performs the following
steps when called:
1. If NewTarget is undefined, throw a
TypeError exception.
2. Let constructorName be the String value of the
Constructor Name value specified
in Table 71 for
this TypedArrayconstructor.
3. Let proto be
"%TypedArray.prototype%".
4. Let numberOfArgs be the number of elements in
args.
The abstract operation AllocateTypedArray takes arguments constructorName (a String which is
the name of a TypedArrayconstructor in
Table 71),
newTarget (a constructor), and
defaultProto (a String) and optional argument length (a non-negative integer)
and returns either a normal completion
containing a TypedArray or a throw
completion. It is used to validate and create an instance of a TypedArrayconstructor. If the
length argument is passed, an ArrayBuffer of that length is also allocated and associated
with the new TypedArray instance.
AllocateTypedArray provides common semantics that is used by TypedArray. It performs the
following steps when called:
23.2.5.1.5 InitializeTypedArrayFromArrayLike ( O,
arrayLike )
The abstract operation InitializeTypedArrayFromArrayLike takes arguments O (a TypedArray) and arrayLike
(an Object, but not a TypedArray or an ArrayBuffer) and
returns either a normal completion
containingunused or a throw
completion. It performs the following steps when called:
The abstract operation AllocateTypedArrayBuffer takes arguments O (a TypedArray) and length (a
non-negative integer) and returns either a normal completion
containingunused or a throw
completion. It allocates and associates an ArrayBuffer with O. It performs
the following steps when called:
does not have a [[ViewedArrayBuffer]] or any other of the internal slots that
are specific to TypedArray instance objects.
23.2.7.1TypedArray.prototype.BYTES_PER_ELEMENT
The value of TypedArray.prototype.BYTES_PER_ELEMENT is the Element Size value
specified in Table 71 for
TypedArray.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.
23.2.7.2TypedArray.prototype.constructor
The initial value of the "constructor" property of the prototype for a given
TypedArrayconstructor is the constructor itself.
23.2.8 Properties of TypedArray Instances
TypedArray instances are TypedArrays. Each TypedArray
instance inherits properties from the corresponding TypedArray prototype object. Each
TypedArray instance has the following internal slots: [[TypedArrayName]], [[ViewedArrayBuffer]], [[ByteLength]], [[ByteOffset]], and [[ArrayLength]].
24 Keyed Collections
24.1 Map Objects
Maps are collections of key/value pairs where both the keys and values may be arbitrary ECMAScript language
values. A distinct key value may only occur in one key/value pair within the Map's
collection. Distinct key values are discriminated using the SameValueZero comparison algorithm.
Maps must be implemented using either hash tables or other mechanisms that, on average, provide access times
that are sublinear on the number of elements in the collection. The data structure used in this specification
is only intended to describe the required observable semantics of Maps. It is not intended to be a viable
implementation model.
is the initial value of the "Map" property of the global object.
creates and initializes a new Map when called as a constructor.
is not intended to be called as a function and will throw an exception when called in that manner.
may be used as the value in an extends clause of a class definition. Subclass constructors that intend to inherit
the specified Map behaviour must include a super call to the Map constructor to create and initialize
the subclass instance with the internal state necessary to support the Map.prototype built-in
methods.
24.1.1.1 Map ( [ iterable ] )
This function performs the following steps when called:
1. If NewTarget is undefined, throw a
TypeError exception.
If the parameter iterable is present, it is expected to be an object that implements an
@@iterator method that
returns an iterator object that produces a two element array-like
object whose first element is a value that will be used as a Map key and whose
second element is the value to associate with that key.
The parameter iterable is expected to be an object that implements an @@iterator method that
returns an iterator object that produces a two element array-like
object whose first element is a value that will be used as a Map key and whose
second element is the value to associate with that key.
callbackfn should be a function that accepts two arguments. groupBy calls
callbackfn once for each element in items, in ascending order, and constructs a
new Map. Each value returned by callbackfn is used as a key in the Map. For each such key,
the result Map has an entry whose key is that key and whose value is an array containing all the
elements for which callbackfn returned that key.
callbackfn is called with two arguments: the value of the element and the index of the
element.
The return value of groupBy is a Map.
This function performs the following steps when called:
1. Let groups be ? GroupBy(items, callbackfn,
zero).
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.
24.1.2.3 get Map [ @@species ]
Map[@@species] is an accessor property whose set
accessor function is undefined. Its get accessor function performs the following steps
when called:
1. Return the this value.
The value of the "name" property of this function is "get
[Symbol.species]".
Note
Methods that create derived collection objects should call @@species
to determine
the constructor to use to create the
derived objects. Subclass constructor may over-ride
@@species to change the
default constructor assignment.
3. For each Record
{ [[Key]], [[Value]] } p of
M.[[MapData]], do
a. Set p.[[Key]] to
empty.
b. Set p.[[Value]] to
empty.
4. Return undefined.
Note
The existing [[MapData]]List is
preserved because there may be existing Map Iterator objects that are suspended midway through
iterating over that List.
24.1.3.2 Map.prototype.constructor
The initial value of Map.prototype.constructor is %Map%.
24.1.3.3 Map.prototype.delete ( key )
This method performs the following steps when called:
3. For each Record
{ [[Key]], [[Value]] } p of
M.[[MapData]], do
a. If p.[[Key]] is not
empty and SameValueZero(p.[[Key]], key) is true, then
i. Set p.[[Key]] to
empty.
ii. Set p.[[Value]] to
empty.
iii. Return true.
4. Return false.
Note
The value empty is used as a specification device to indicate that an entry
has been deleted. Actual implementations may take other actions such as physically removing the entry
from internal data structures.
24.1.3.4 Map.prototype.entries ( )
This method performs the following steps when called:
3. If IsCallable(callbackfn)
is false, throw a TypeError exception.
4. Let entries be M.[[MapData]].
5. Let numEntries be the number of elements in
entries.
6. Let index be 0.
7. Repeat, while index < numEntries,
a. Let e be entries[index].
b. Set index to index + 1.
c. If e.[[Key]] is not
empty, then
i. Perform ? Call(callbackfn, thisArg, «
e.[[Value]], e.[[Key]], M »).
ii. NOTE: The number of elements in entries may
have increased during execution of callbackfn.
iii. Set numEntries to the number of elements in
entries.
8. Return undefined.
Note
callbackfn should be a function that accepts three arguments. forEach calls
callbackfn once for each key/value pair present in the Map, in key insertion order.
callbackfn is called only for keys of the Map which actually exist; it is not called for
keys that have been deleted from the Map.
If a thisArg parameter is provided, it will be used as the this value
for each invocation of callbackfn. If it is not provided, undefined is
used instead.
callbackfn is called with three arguments: the value of the item, the key of the item, and
the Map being traversed.
forEach does not directly mutate the object on which it is called but the object may be
mutated by the calls to callbackfn. Each entry of a map's [[MapData]] is only visited once. New keys added after the call to
forEach begins are visited. A key will be revisited if it is deleted after it has been
visited and then re-added before the forEach call completes. Keys that are deleted after
the call to forEach begins and before being visited are not visited unless the key is
added again before the forEach call completes.
24.1.3.6 Map.prototype.get ( key )
This method performs the following steps when called:
3. For each Record
{ [[Key]], [[Value]] } p of
M.[[MapData]], do
a. If p.[[Key]] is not
empty and SameValueZero(p.[[Key]], key) is true, then
i. Set p.[[Value]] to
value.
ii. Return M.
4. If key is -0𝔽, set
key to +0𝔽.
5. Let p be the Record
{ [[Key]]: key, [[Value]]:
value }.
6. Append p to M.[[MapData]].
7. Return M.
24.1.3.10 get Map.prototype.size
Map.prototype.size is an accessor property whose set
accessor function is undefined. Its get accessor function performs the following steps
when called:
The initial value of the @@iterator property is
%Map.prototype.entries%, defined in 24.1.3.4.
24.1.3.13 Map.prototype [ @@toStringTag ]
The initial value of the @@toStringTag property is
the String value "Map".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
true }.
24.1.4 Properties of Map Instances
Map instances are ordinary objects that inherit
properties from the Map prototype. Map instances also have a [[MapData]] internal
slot.
24.1.5 Map Iterator Objects
A Map Iterator is an object, that represents a specific iteration over some specific Map instance object.
There is not a named constructor for Map Iterator objects.
Instead, map iterator objects are created by calling certain methods of Map instance objects.
24.1.5.1 CreateMapIterator ( map, kind )
The abstract operation CreateMapIterator takes arguments map (an ECMAScript language
value) and kind (key+value,
key, or value) and returns either a normal completion
containing a Generator or a throw
completion. It is used to create iterator objects for Map methods that return such
iterators. It performs the following steps when called:
The initial value of the @@toStringTag property is
the String value "Map Iterator".
This property has the attributes { [[Writable]]: false,
[[Enumerable]]: false, [[Configurable]]: true }.
24.2 Set Objects
Set objects are collections of ECMAScript language
values. A distinct value may only occur once as an element of a Set's collection. Distinct
values are discriminated using the SameValueZero comparison algorithm.
Set objects must be implemented using either hash tables or other mechanisms that, on average, provide access
times that are sublinear on the number of elements in the collection. The data structure used in this
specification is only intended to describe the required observable semantics of Set objects. It is not
intended to be a viable implementation model.
is the initial value of the "Set" property of the global object.
creates and initializes a new Set object when called as a constructor.
is not intended to be called as a function and will throw an exception when called in that manner.
may be used as the value in an extends clause of a class definition. Subclass constructors that intend to inherit
the specified Set behaviour must include a super call to the Set constructor to create and initialize
the subclass instance with the internal state necessary to support the Set.prototype built-in
methods.
24.2.1.1 Set ( [ iterable ] )
This function performs the following steps when called:
1. If NewTarget is undefined, throw a
TypeError exception.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.
24.2.2.2 get Set [ @@species ]
Set[@@species] is an accessor property whose set
accessor function is undefined. Its get accessor function performs the following steps
when called:
1. Return the this value.
The value of the "name" property of this function is "get
[Symbol.species]".
Note
Methods that create derived collection objects should call @@species
to determine
the constructor to use to create the
derived objects. Subclass constructor may over-ride
@@species to change the
default constructor assignment.
a. Replace the element of S.[[SetData]] whose value is e with an element whose value is
empty.
4. Return undefined.
Note
The existing [[SetData]]List is
preserved because there may be existing Set Iterator objects that are suspended midway through
iterating over that List.
24.2.3.3 Set.prototype.constructor
The initial value of Set.prototype.constructor is %Set%.
24.2.3.4 Set.prototype.delete ( value )
This method performs the following steps when called:
a. If e is not empty and
SameValueZero(e,
value) is true, then
i. Replace the element of S.[[SetData]] whose value is e with an element whose value is
empty.
ii. Return true.
4. Return false.
Note
The value empty is used as a specification device to indicate that an entry
has been deleted. Actual implementations may take other actions such as physically removing the entry
from internal data structures.
24.2.3.5 Set.prototype.entries ( )
This method performs the following steps when called:
3. If IsCallable(callbackfn)
is false, throw a TypeError exception.
4. Let entries be S.[[SetData]].
5. Let numEntries be the number of elements in
entries.
6. Let index be 0.
7. Repeat, while index < numEntries,
a. Let e be entries[index].
b. Set index to index + 1.
c. If e is not empty, then
i. Perform ? Call(callbackfn, thisArg, «
e, e, S »).
ii. NOTE: The number of elements in entries may
have increased during execution of callbackfn.
iii. Set numEntries to the number of elements in
entries.
8. Return undefined.
Note
callbackfn should be a function that accepts three arguments. forEach calls
callbackfn once for each value present in the Set object, in value insertion order.
callbackfn is called only for values of the Set which actually exist; it is not called for
keys that have been deleted from the set.
If a thisArg parameter is provided, it will be used as the this value
for each invocation of callbackfn. If it is not provided, undefined is
used instead.
callbackfn is called with three arguments: the first two arguments are a value contained
in the Set. The same value is passed for both arguments. The Set object being traversed is passed as
the third argument.
The callbackfn is called with three arguments to be consistent with the call back
functions used by forEach methods for Map and Array. For Sets, each item value is
considered to be both the key and the value.
forEach does not directly mutate the object on which it is called but the object may be
mutated by the calls to callbackfn.
Each value is normally visited only once. However, a value will be revisited if it is deleted after
it has been visited and then re-added before the forEach call completes. Values that are
deleted after the call to forEach begins and before being visited are not visited unless
the value is added again before the forEach call completes. New values added after the
call to forEach begins are visited.
24.2.3.7 Set.prototype.has ( value )
This method performs the following steps when called:
a. If e is not empty and
SameValueZero(e,
value) is true, return true.
4. Return false.
24.2.3.8 Set.prototype.keys ( )
The initial value of the "keys" property is %Set.prototype.values%, defined in
24.2.3.10.
Note
For iteration purposes, a Set appears similar to a Map where each entry has the same value for its
key and value.
24.2.3.9 get Set.prototype.size
Set.prototype.size is an accessor property whose set
accessor function is undefined. Its get accessor function performs the following steps
when called:
The initial value of the @@iterator property is
%Set.prototype.values%, defined in 24.2.3.10.
24.2.3.12 Set.prototype [ @@toStringTag ]
The initial value of the @@toStringTag property is
the String value "Set".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
true }.
24.2.4 Properties of Set Instances
Set instances are ordinary objects that inherit
properties from the Set prototype. Set instances also have a [[SetData]] internal
slot.
24.2.5 Set Iterator Objects
A Set Iterator is an ordinary object, with the structure
defined below, that represents a specific iteration over some specific Set instance object. There is not a
named constructor for Set Iterator objects.
Instead, set iterator objects are created by calling certain methods of Set instance objects.
24.2.5.1 CreateSetIterator ( set, kind )
The abstract operation CreateSetIterator takes arguments set (an ECMAScript language
value) and kind (key+value or
value) and returns either a normal completion
containing a Generator or a throw
completion. It is used to create iterator objects for Set methods that return such
iterators. It performs the following steps when called:
The initial value of the @@toStringTag property is
the String value "Set Iterator".
This property has the attributes { [[Writable]]: false,
[[Enumerable]]: false, [[Configurable]]: true }.
24.3 WeakMap Objects
WeakMaps are collections of key/value pairs where the keys are objects and/or symbols and values may be
arbitrary ECMAScript language
values. A WeakMap may be queried to see if it contains a key/value pair with a specific
key, but no mechanism is provided for enumerating the values it holds as keys. In certain conditions, values
which are not live are removed as WeakMap keys, as
described in 9.10.3.
An implementation may impose an arbitrarily determined latency between the time a key/value pair of a WeakMap
becomes inaccessible and the time when the key/value pair is removed from the WeakMap. If this latency was
observable to ECMAScript program, it would be a source of indeterminacy that could impact program execution.
For that reason, an ECMAScript implementation must not provide any means to observe a key of a WeakMap that
does not require the observer to present the observed key.
WeakMaps must be implemented using either hash tables or other mechanisms that, on average, provide access
times that are sublinear on the number of key/value pairs in the collection. The data structure used in this
specification is only intended to describe the required observable semantics of WeakMaps. It is not intended
to be a viable implementation model.
Note
WeakMap and WeakSet are intended to provide mechanisms for dynamically associating state with an object
or symbol in a manner that does not “leak” memory resources if, in the absence of the WeakMap or WeakSet
instance, the object or symbol otherwise became inaccessible and subject to resource reclamation by the
implementation's garbage collection mechanisms. This characteristic can be achieved by using an inverted
per-object/symbol mapping of WeakMap or WeakSet instances to keys. Alternatively, each WeakMap or WeakSet
instance may internally store its key and value data, but this approach requires coordination between the
WeakMap or WeakSet implementation and the garbage collector. The following references describe mechanism
that may be useful to implementations of WeakMap and WeakSet:
Barry Hayes. 1997. Ephemerons: a new finalization mechanism. In Proceedings of the 12th ACM SIGPLAN
conference on Object-oriented programming, systems, languages, and applications (OOPSLA '97), A.
Michael Berman (Ed.). ACM, New York, NY, USA, 176-183, http://doi.acm.org/10.1145/263698.263733.
is the initial value of the "WeakMap" property of the global
object.
creates and initializes a new WeakMap when called as a constructor.
is not intended to be called as a function and will throw an exception when called in that manner.
may be used as the value in an extends clause of a class definition. Subclass constructors that intend to inherit
the specified WeakMap behaviour must include a super call to the WeakMap constructor to create and initialize
the subclass instance with the internal state necessary to support the WeakMap.prototype
built-in methods.
24.3.1.1 WeakMap ( [ iterable ] )
This function performs the following steps when called:
1. If NewTarget is undefined, throw a
TypeError exception.
If the parameter iterable is present, it is expected to be an object that implements an
@@iterator method that
returns an iterator object that produces a two element array-like
object whose first element is a value that will be used as a WeakMap key and whose
second element is the value to associate with that key.
4. For each Record
{ [[Key]], [[Value]] } p of
M.[[WeakMapData]], do
a. If p.[[Key]] is not
empty and SameValue(p.[[Key]], key) is true, then
i. Set p.[[Key]] to
empty.
ii. Set p.[[Value]] to
empty.
iii. Return true.
5. Return false.
Note
The value empty is used as a specification device to indicate that an entry
has been deleted. Actual implementations may take other actions such as physically removing the entry
from internal data structures.
24.3.3.3 WeakMap.prototype.get ( key )
This method performs the following steps when called:
3. If CanBeHeldWeakly(key)
is false, throw a TypeError exception.
4. For each Record
{ [[Key]], [[Value]] } p of
M.[[WeakMapData]], do
a. If p.[[Key]] is not
empty and SameValue(p.[[Key]], key) is true, then
i. Set p.[[Value]] to
value.
ii. Return M.
5. Let p be the Record
{ [[Key]]: key, [[Value]]:
value }.
6. Append p to M.[[WeakMapData]].
7. Return M.
24.3.3.6 WeakMap.prototype [ @@toStringTag ]
The initial value of the @@toStringTag property is
the String value "WeakMap".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
true }.
24.3.4 Properties of WeakMap Instances
WeakMap instances are ordinary objects that inherit
properties from the WeakMap prototype. WeakMap instances also have a [[WeakMapData]] internal slot.
24.4 WeakSet Objects
WeakSets are collections of objects and/or symbols. A distinct object or symbol may only occur once as an
element of a WeakSet's collection. A WeakSet may be queried to see if it contains a specific value, but no
mechanism is provided for enumerating the values it holds. In certain conditions, values which are not
live are removed as WeakSet elements, as
described in 9.10.3.
An implementation may impose an arbitrarily determined latency between the time a value contained in a
WeakSet becomes inaccessible and the time when the value is removed from the WeakSet. If this latency was
observable to ECMAScript program, it would be a source of indeterminacy that could impact program execution.
For that reason, an ECMAScript implementation must not provide any means to determine if a WeakSet contains a
particular value that does not require the observer to present the observed value.
WeakSets must be implemented using either hash tables or other mechanisms that, on average, provide access
times that are sublinear on the number of elements in the collection. The data structure used in this
specification is only intended to describe the required observable semantics of WeakSets. It is not intended
to be a viable implementation model.
is the initial value of the "WeakSet" property of the global
object.
creates and initializes a new WeakSet when called as a constructor.
is not intended to be called as a function and will throw an exception when called in that manner.
may be used as the value in an extends clause of a class definition. Subclass constructors that intend to inherit
the specified WeakSet behaviour must include a super call to the WeakSet constructor to create and initialize
the subclass instance with the internal state necessary to support the WeakSet.prototype
built-in methods.
24.4.1.1 WeakSet ( [ iterable ] )
This function performs the following steps when called:
1. If NewTarget is undefined, throw a
TypeError exception.
a. If e is not empty and
SameValue(e,
value) is true, then
i. Replace the element of S.[[WeakSetData]] whose value is e with an element whose value
is empty.
ii. Return true.
5. Return false.
Note
The value empty is used as a specification device to indicate that an entry
has been deleted. Actual implementations may take other actions such as physically removing the entry
from internal data structures.
24.4.3.4 WeakSet.prototype.has ( value )
This method performs the following steps when called:
a. If e is not empty and
SameValue(e,
value) is true, return true.
5. Return false.
24.4.3.5 WeakSet.prototype [ @@toStringTag ]
The initial value of the @@toStringTag property is
the String value "WeakSet".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
true }.
24.4.4 Properties of WeakSet Instances
WeakSet instances are ordinary objects that inherit
properties from the WeakSet prototype. WeakSet instances also have a [[WeakSetData]] internal slot.
25 Structured Data
25.1 ArrayBuffer Objects
25.1.1 Notation
The descriptions below in this section, 25.4, and 29 use the read-modify-write
modification function internal data structure.
A read-modify-write modification
function is a mathematical function that is notationally represented as an abstract closure that
takes two Lists of
byte
values as arguments and returns
a List of
byte
values. These abstract closures
satisfy all of the following properties:
They perform all their algorithm steps atomically.
Their individual algorithm steps are not observable.
Note
To aid verifying that a read-modify-write modification function's algorithm steps constitute a pure,
mathematical function, the following editorial conventions are recommended:
They do not access, directly or transitively via invoked abstract
operations and abstract closures, any language or specification values except their
parameters and captured values.
The abstract operation AllocateArrayBuffer takes arguments constructor (a constructor) and byteLength
(a non-negative integer) and optional argument
maxByteLength (a non-negative integer or empty)
and returns either a normal completion
containing an ArrayBuffer or a throw
completion. It is used to create an ArrayBuffer. It performs the following steps when
called:
1. Let slots be « [[ArrayBufferData]], [[ArrayBufferByteLength]], [[ArrayBufferDetachKey]] ».
2. If maxByteLength is present and
maxByteLength is not empty, let allocatingResizableBuffer
be true; otherwise let allocatingResizableBuffer be
false.
3. If allocatingResizableBuffer is
true, then
a. If byteLength > maxByteLength, throw
a RangeError exception.
7. Set obj.[[ArrayBufferByteLength]] to byteLength.
8. If allocatingResizableBuffer is
true, then
a. If it is not possible to create a Data
Blockblock consisting of maxByteLength bytes, throw a
RangeError exception.
b. NOTE: Resizable ArrayBuffers are designed to be implementable
with in-place growth. Implementations may throw if, for example, virtual memory cannot be reserved
up front.
c. Set obj.[[ArrayBufferMaxByteLength]] to maxByteLength.
9. Return obj.
25.1.3.2 ArrayBufferByteLength ( arrayBuffer, order )
The abstract operation ArrayBufferByteLength takes arguments arrayBuffer (an ArrayBuffer or
SharedArrayBuffer) and order (seq-cst or
unordered) and returns a non-negative integer. It performs the following steps
when called:
1. If IsSharedArrayBuffer(arrayBuffer)
is true and arrayBuffer has an [[ArrayBufferByteLengthData]] internal slot, then
a. Let bufferByteLengthBlock be
arrayBuffer.[[ArrayBufferByteLengthData]].
14. NOTE: Neither creation of the new Data
Block nor copying from the old Data Block are observable.
Implementations may implement this method as a zero-copy move or a realloc.
The abstract operation IsDetachedBuffer takes argument arrayBuffer (an ArrayBuffer or a
SharedArrayBuffer) and returns a Boolean. It performs the following steps when called:
1. If arrayBuffer.[[ArrayBufferData]] is null, return true.
The abstract operation DetachArrayBuffer takes argument arrayBuffer (an ArrayBuffer) and
optional argument key (anything) and returns either a normal completion
containingunused or a throw
completion. It performs the following steps when called:
3. If arrayBuffer.[[ArrayBufferDetachKey]] is not key, throw a
TypeError exception.
4. Set arrayBuffer.[[ArrayBufferData]] to null.
5. Set arrayBuffer.[[ArrayBufferByteLength]] to 0.
6. Return unused.
Note
Detaching an ArrayBuffer instance disassociates the Data Block
used as its backing store from the instance and sets the byte length of the buffer to 0.
The abstract operation CloneArrayBuffer takes arguments srcBuffer (an ArrayBuffer or a
SharedArrayBuffer), srcByteOffset (a non-negative integer), and srcLength (a
non-negative integer) and returns either a normal completion
containing an ArrayBuffer or a throw
completion. It creates a new ArrayBuffer whose data is a copy of srcBuffer's
data over the range starting at srcByteOffset and continuing for srcLength bytes. It
performs the following steps when called:
The host-defined abstract operation
HostResizeArrayBuffer takes arguments buffer (an ArrayBuffer) and newByteLength (a
non-negative integer) and returns either a normal completion
containing either handled or unhandled,
or a throw
completion. It gives the host an opportunity to perform implementation-defined
resizing of buffer. If the host chooses not to handle resizing of
buffer, it may return unhandled for the default behaviour.
The implementation of HostResizeArrayBuffer must conform to the following requirements:
The abstract operation does not detach buffer.
If the abstract operation completes normally with handled,
buffer.[[ArrayBufferByteLength]] is newByteLength.
The default implementation of HostResizeArrayBuffer is to return NormalCompletion(unhandled).
25.1.3.9 IsFixedLengthArrayBuffer ( arrayBuffer )
The abstract operation IsFixedLengthArrayBuffer takes argument arrayBuffer (an ArrayBuffer or
a SharedArrayBuffer) and returns a Boolean. It performs the following steps when called:
1. If arrayBuffer has an [[ArrayBufferMaxByteLength]] internal slot, return false.
2. Return true.
25.1.3.10 IsUnsignedElementType ( type )
The abstract operation IsUnsignedElementType takes argument type (a TypedArray element type)
and returns a Boolean. It verifies if the argument type is an unsigned TypedArray element type. It
performs the following steps when called:
1. If type is one of uint8,
uint8clamped, uint16, uint32, or
biguint64, return true.
2. Return false.
25.1.3.11 IsUnclampedIntegerElementType ( type )
The abstract operation IsUnclampedIntegerElementType takes argument type (a TypedArray element type)
and returns a Boolean. It verifies if the argument type is an IntegerTypedArray element type not
including uint8clamped. It performs the following steps when called:
1. If type is one of int8,
uint8, int16, uint16,
int32, or uint32, return true.
1. If type is either biguint64 or
bigint64, return true.
2. Return false.
25.1.3.13 IsNoTearConfiguration ( type, order )
The abstract operation IsNoTearConfiguration takes arguments type (a TypedArray element type)
and order (seq-cst, unordered, or
init) and returns a Boolean. It performs the following steps when called:
The abstract operation RawBytesToNumeric takes arguments type (a TypedArray element type),
rawBytes (a List of
byte
values), and
isLittleEndian (a Boolean) and returns a Number or a BigInt. It performs the following steps
when called:
1. Let elementSize be the Element Size value specified in
Table 71 for
Element Type type.
2. If isLittleEndian is false, reverse
the order of the elements of rawBytes.
3. If type is float32, then
a. Let value be the byte elements of
rawBytes concatenated and interpreted as a little-endian bit string encoding of an
IEEE 754-2019 binary32
value.
b. If value is an IEEE
754-2019 binary32 NaN value, return the NaN Number value.
c. Return the Number value that corresponds to value.
4. If type is float64, then
a. Let value be the byte elements of
rawBytes concatenated and interpreted as a little-endian bit string encoding of an
IEEE 754-2019 binary64
value.
b. If value is an IEEE
754-2019 binary64 NaN value, return the NaN Number value.
c. Return the Number value that corresponds to value.
a. Let intValue be the byte elements of
rawBytes concatenated and interpreted as a bit string encoding of an unsigned
little-endian binary number.
6. Else,
a. Let intValue be the byte elements of
rawBytes concatenated and interpreted as a bit string encoding of a binary
little-endian two's complement number of bit length elementSize × 8.
7. If IsBigIntElementType(type)
is true, return the BigInt value that corresponds to intValue.
8. Otherwise, return the Number value that corresponds to
intValue.
25.1.3.15 GetRawBytesFromSharedBlock ( block,
byteIndex, type, isTypedArray, order )
The abstract operation GetRawBytesFromSharedBlock takes arguments block (a Shared
Data Block), byteIndex (a non-negative integer), type
(a TypedArray element type),
isTypedArray (a Boolean), and order (seq-cst or
unordered) and returns a List of
byte
values. It performs the
following steps when called:
1. Let elementSize be the Element Size value specified in
Table 71 for
Element Type type.
4. If isTypedArray is true and
IsNoTearConfiguration(type,
order) is true, let noTear be true;
otherwise let noTear be false.
5. Let rawValue be a List of
length elementSize whose elements are nondeterministically chosen byte
values.
6. NOTE: In implementations, rawValue is the result of a
non-atomic or atomic read instruction on the underlying hardware. The nondeterminism is a semantic
prescription of the memory model to describe
observable behaviour of hardware with weak consistency.
7. Let readEvent be ReadSharedMemory
{ [[Order]]: order, [[NoTear]]:
noTear, [[Block]]: block, [[ByteIndex]]: byteIndex, [[ElementSize]]:
elementSize }.
8. Append readEvent to eventsRecord.[[EventList]].
9. Append Chosen Value
Record { [[Event]]: readEvent, [[ChosenValue]]: rawValue } to execution.[[ChosenValues]].
The abstract operation GetValueFromBuffer takes arguments arrayBuffer (an ArrayBuffer or
SharedArrayBuffer), byteIndex (a non-negative integer), type (a TypedArray element type),
isTypedArray (a Boolean), and order (seq-cst or
unordered) and optional argument isLittleEndian (a Boolean) and returns
a Number or a BigInt. It performs the following steps when called:
a. Let rawValue be a List
whose elements are bytes from block at indices in the interval from
byteIndex (inclusive) to byteIndex + elementSize (exclusive).
7. Assert: The number of elements in
rawValue is elementSize.
8. If isLittleEndian is not present, set
isLittleEndian to the value of the [[LittleEndian]] field of the
surrounding agent's
Agent
Record.
The abstract operation NumericToRawBytes takes arguments type (a TypedArray element type),
value (a Number or a BigInt), and isLittleEndian (a Boolean) and returns a List of
byte
values. It performs the
following steps when called:
1. If type is float32, then
a. Let rawBytes be a List
whose elements are the 4 bytes that are the result of converting value to IEEE 754-2019 binary32
format using roundTiesToEven mode. The bytes are arranged in little endian order. If
value is NaN, rawBytes may be set to any implementation
chosen IEEE 754-2019 binary32
format Not-a-Number encoding. An implementation must always choose the same encoding for each
implementation distinguishable NaN value.
2. Else if type is float64, then
a. Let rawBytes be a List
whose elements are the 8 bytes that are the IEEE 754-2019 binary64
format encoding of value. The bytes are arranged in little endian order. If
value is NaN, rawBytes may be set to any implementation
chosen IEEE 754-2019 binary64
format Not-a-Number encoding. An implementation must always choose the same encoding for each
implementation distinguishable NaN value.
3. Else,
a. Let n be the Element Size value specified in
Table 71
for Element Type type.
b. Let convOp be the abstract operation named in the
Conversion Operation column in Table
71 for Element Type type.
The abstract operation SetValueInBuffer takes arguments arrayBuffer (an ArrayBuffer or
SharedArrayBuffer), byteIndex (a non-negative integer), type (a TypedArray element type),
value (a Number or a BigInt), isTypedArray (a Boolean), and order
(seq-cst, unordered, or init) and
optional argument isLittleEndian (a Boolean) and returns unused. It
performs the following steps when called:
c. If isTypedArray is true and
IsNoTearConfiguration(type,
order) is true, let noTear be true;
otherwise let noTear be false.
d. Append WriteSharedMemory
{ [[Order]]: order, [[NoTear]]:
noTear, [[Block]]: block, [[ByteIndex]]: byteIndex, [[ElementSize]]: elementSize, [[Payload]]: rawBytes } to eventsRecord.[[EventList]].
9. Else,
a. Store the individual bytes of rawBytes into
block, starting at block[byteIndex].
10. Return unused.
25.1.3.19 GetModifySetValueInBuffer ( arrayBuffer,
byteIndex, type, value, op )
The abstract operation GetModifySetValueInBuffer takes arguments arrayBuffer (an ArrayBuffer
or a SharedArrayBuffer), byteIndex (a non-negative integer), type (a TypedArray element type),
value (a Number or a BigInt), and op (a read-modify-write
modification function) and returns a Number or a BigInt. It performs the following
steps when called:
c. Let rawBytesRead be a List
of length elementSize whose elements are nondeterministically chosen byte
values.
d. NOTE: In implementations, rawBytesRead is the
result of a load-link, of a load-exclusive, or of an operand of a read-modify-write instruction on
the underlying hardware. The nondeterminism is a semantic prescription of the memory model to describe
observable behaviour of hardware with weak consistency.
e. Let rmwEvent be ReadModifyWriteSharedMemory
{ [[Order]]: seq-cst, [[NoTear]]: true, [[Block]]:
block, [[ByteIndex]]: byteIndex, [[ElementSize]]: elementSize, [[Payload]]: rawBytes, [[ModifyOp]]:
op }.
f. Append rmwEvent to eventsRecord.[[EventList]].
g. Append Chosen Value
Record { [[Event]]: rmwEvent, [[ChosenValue]]: rawBytesRead } to execution.[[ChosenValues]].
9. Else,
a. Let rawBytesRead be a List
of length elementSize whose elements are the sequence of elementSize bytes
starting with block[byteIndex].
b. Let rawBytesModified be
op(rawBytesRead, rawBytes).
c. Store the individual bytes of rawBytesModified
into block, starting at block[byteIndex].
is the initial value of the "ArrayBuffer" property of the global
object.
creates and initializes a new ArrayBuffer when called as a constructor.
is not intended to be called as a function and will throw an exception when called in that manner.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit
the specified ArrayBuffer behaviour must include a super call to the ArrayBuffer constructor to create and initialize
subclass instances with the internal state necessary to support the ArrayBuffer.prototype
built-in methods.
25.1.4.1 ArrayBuffer ( length [ , options ] )
This function performs the following steps when called:
1. If NewTarget is undefined, throw a
TypeError exception.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.
25.1.5.3 get ArrayBuffer [ @@species ]
ArrayBuffer[@@species] is an accessor property whose set
accessor function is undefined. Its get accessor function performs the following steps
when called:
1. Return the this value.
The value of the "name" property of this function is "get
[Symbol.species]".
does not have an [[ArrayBufferData]] or [[ArrayBufferByteLength]] internal slot.
25.1.6.1 get ArrayBuffer.prototype.byteLength
ArrayBuffer.prototype.byteLength is an accessor property whose set
accessor function is undefined. Its get accessor function performs the following steps
when called:
The initial value of ArrayBuffer.prototype.constructor is %ArrayBuffer%.
25.1.6.3 get ArrayBuffer.prototype.detached
ArrayBuffer.prototype.detached is an accessor property whose set
accessor function is undefined. Its get accessor function performs the following steps
when called:
ArrayBuffer.prototype.maxByteLength is an accessor property
whose set accessor function is undefined. Its get accessor function performs the
following steps when called:
ArrayBuffer.prototype.resizable is an accessor property whose set
accessor function is undefined. Its get accessor function performs the following steps
when called:
13. NOTE: Neither creation of the new Data
Block nor copying from the old Data Block are observable.
Implementations may implement this method as in-place growth or shrinkage.
14. Set O.[[ArrayBufferData]] to
newBlock.
15. Set O.[[ArrayBufferByteLength]] to newByteLength.
16. Return undefined.
25.1.6.7 ArrayBuffer.prototype.slice ( start, end )
This method performs the following steps when called:
The initial value of the @@toStringTag property is
the String value "ArrayBuffer".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
true }.
25.1.7 Properties of ArrayBuffer Instances
ArrayBuffer instances inherit properties from the ArrayBuffer
prototype object. ArrayBuffer instances each have an [[ArrayBufferData]] internal slot, an [[ArrayBufferByteLength]] internal slot, and an [[ArrayBufferDetachKey]] internal slot. ArrayBuffer instances which are resizable each
have an [[ArrayBufferMaxByteLength]] internal slot.
ArrayBuffer instances whose [[ArrayBufferData]] is null are
considered to be detached and all operators to access or modify data contained in the ArrayBuffer instance
will fail.
ArrayBuffer instances whose [[ArrayBufferDetachKey]] is set to a value other than
undefined need to have all DetachArrayBuffer calls
passing that same "detach key" as an argument, otherwise a TypeError will result. This internal slot is only
ever set by certain embedding environments, not by algorithms in this specification.
25.1.8 Resizable ArrayBuffer Guidelines
Note 1
The following are guidelines for ECMAScript programmers working with resizable
ArrayBuffer.
We recommend that programs be tested in their deployment environments where possible. The amount of
available physical memory differs greatly between hardware devices. Similarly, virtual memory subsystems
also differ greatly between hardware devices as well as operating systems. An application that runs
without out-of-memory errors on a 64-bit desktop web browser could run out of memory on a 32-bit mobile
web browser.
When choosing a value for the "maxByteLength" option for resizable
ArrayBuffer, we recommend that the smallest possible size for the application be
chosen. We recommend that "maxByteLength" does not exceed 1,073,741,824 (2**30 bytes or 1GiB).
Please note that successfully constructing a resizable
ArrayBuffer for a particular maximum size does not guarantee that future resizes will
succeed.
Note 2
The following are guidelines for ECMAScript implementers implementing resizable
ArrayBuffer.
Resizable
ArrayBuffer can be implemented as copying upon resize, as in-place growth via
reserving virtual memory up front, or as a combination of both for different values of the constructor's
"maxByteLength" option.
If a host is multi-tenanted (i.e. it runs many
ECMAScript applications simultaneously), such as a web browser, and its implementations choose to
implement in-place growth by reserving virtual memory, we recommend that both 32-bit and 64-bit
implementations throw for values of "maxByteLength" ≥ 1GiB to 1.5GiB. This is to
reduce the likelihood a single application can exhaust the virtual memory address space and to reduce
interoperability risk.
If a host does not have virtual memory, such as
those running on embedded devices without an MMU, or if a host only implements resizing by copying,
it may accept any Number value for the "maxByteLength"
option. However, we recommend a RangeError be thrown if a memory block of the
requested size can never be allocated. For example, if the requested size is greater than the maximium
amount of usable memory on the device.
25.2 SharedArrayBuffer Objects
25.2.1 Fixed-length and Growable SharedArrayBuffer Objects
A fixed-length SharedArrayBuffer is a SharedArrayBuffer whose byte length cannot
change after creation.
The abstract operation AllocateSharedArrayBuffer takes arguments constructor (a constructor) and byteLength
(a non-negative integer) and optional argument
maxByteLength (a non-negative integer or empty)
and returns either a normal completion
containing a SharedArrayBuffer or a throw
completion. It is used to create a SharedArrayBuffer. It performs the following steps
when called:
1. Let slots be « [[ArrayBufferData]] ».
2. If maxByteLength is present and
maxByteLength is not empty, let allocatingGrowableBuffer
be true; otherwise let allocatingGrowableBuffer be
false.
3. If allocatingGrowableBuffer is
true, then
a. If byteLength > maxByteLength, throw
a RangeError exception.
b. Append [[ArrayBufferByteLengthData]]
and [[ArrayBufferMaxByteLength]] to slots.
c. Perform SetValueInBuffer(byteLengthBlock,
0, biguint64, ℤ(byteLength),
true, seq-cst).
d. Set obj.[[ArrayBufferByteLengthData]] to byteLengthBlock.
e. Set obj.[[ArrayBufferMaxByteLength]] to maxByteLength.
10. Else,
a. Set obj.[[ArrayBufferByteLength]] to byteLength.
11. Return obj.
25.2.2.2 IsSharedArrayBuffer ( obj )
The abstract operation IsSharedArrayBuffer takes argument obj (an ArrayBuffer or a
SharedArrayBuffer) and returns a Boolean. It tests whether an object is an ArrayBuffer, a
SharedArrayBuffer, or a subtype of either. It performs the following steps when called:
The host-defined abstract operation
HostGrowSharedArrayBuffer takes arguments buffer (a SharedArrayBuffer) and
newByteLength (a non-negative integer) and returns either a normal completion
containing either handled or unhandled,
or a throw
completion. It gives the host an opportunity to perform implementation-defined
growing of buffer. If the host chooses not to handle growing of
buffer, it may return unhandled for the default behaviour.
The implementation of HostGrowSharedArrayBuffer must conform to the following requirements:
If the abstract operation does not complete normally with unhandled, and
newByteLength < the current byte length of the buffer or
newByteLength > buffer.[[ArrayBufferMaxByteLength]],
throw a RangeError exception.
Let isLittleEndian be the value of the [[LittleEndian]] field of
the surrounding agent's Agent
Record. If the abstract operation completes normally with
handled, a WriteSharedMemory
or ReadModifyWriteSharedMemory
event whose [[Order]] is seq-cst, [[Payload]] is NumericToRawBytes(biguint64,
newByteLength, isLittleEndian), [[Block]] is
buffer.[[ArrayBufferByteLengthData]], [[ByteIndex]] is 0, and [[ElementSize]] is 8 is added to
the surrounding agent's candidate execution
such that racing calls to SharedArrayBuffer.prototype.grow are not "lost", i.e. silently do
nothing.
Note
The second requirement above is intentionally vague about how or when the current byte length of
buffer is read. Because the byte length must be updated via an atomic read-modify-write
operation on the underlying hardware, architectures that use load-link/store-conditional or
load-exclusive/store-exclusive instruction pairs may wish to keep the paired instructions close in the
instruction stream. As such, SharedArrayBuffer.prototype.grow itself does not perform bounds checking
on newByteLength before calling HostGrowSharedArrayBuffer, nor is there a requirement on
when the current byte length is read.
This is in contrast with HostResizeArrayBuffer,
which is guaranteed that the value of newByteLength is ≥ 0 and ≤ buffer.[[ArrayBufferMaxByteLength]].
The default implementation of HostGrowSharedArrayBuffer is to return NormalCompletion(unhandled).
is the initial value of the "SharedArrayBuffer" property of the global
object, if that property
is present (see below).
creates and initializes a new SharedArrayBuffer when called as a constructor.
is not intended to be called as a function and will throw an exception when called in that manner.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit
the specified SharedArrayBuffer behaviour must include a super call to the SharedArrayBuffer
constructor to create and initialize
subclass instances with the internal state necessary to support the
SharedArrayBuffer.prototype built-in methods.
Whenever a host does not provide concurrent access to
SharedArrayBuffers it may omit the "SharedArrayBuffer" property of the global
object.
Note
Unlike an ArrayBuffer, a SharedArrayBuffer cannot become detached, and its
internal [[ArrayBufferData]] slot is never null.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.
25.2.4.2 get SharedArrayBuffer [ @@species ]
SharedArrayBuffer[@@species] is an accessor property whose set
accessor function is undefined. Its get accessor function performs the following steps
when called:
1. Return the this value.
The value of the "name" property of this function is "get
[Symbol.species]".
25.2.5 Properties of the SharedArrayBuffer Prototype Object
does not have an [[ArrayBufferData]] or [[ArrayBufferByteLength]] internal slot.
25.2.5.1 get SharedArrayBuffer.prototype.byteLength
SharedArrayBuffer.prototype.byteLength is an accessor property
whose set accessor function is undefined. Its get accessor function performs the
following steps when called:
8. Let byteLengthBlock be O.[[ArrayBufferByteLengthData]].
9. Let currentByteLengthRawBytes be GetRawBytesFromSharedBlock(byteLengthBlock,
0, biguint64, true, seq-cst).
10. Let newByteLengthRawBytes be NumericToRawBytes(biguint64,
ℤ(newByteLength),
isLittleEndian).
11. Repeat,
a. NOTE: This is a compare-and-exchange loop to ensure that
parallel, racing grows of the same buffer are totally ordered, are not lost, and do not silently
do nothing. The loop exits if it was able to attempt to grow uncontended.
b. Let currentByteLength be ℝ(RawBytesToNumeric(biguint64,
currentByteLengthRawBytes, isLittleEndian)).
c. If newByteLength = currentByteLength,
return undefined.
d. If newByteLength < currentByteLength
or newByteLength > O.[[ArrayBufferMaxByteLength]], throw a RangeError
exception.
e. Let byteLengthDelta be newByteLength -
currentByteLength.
f. If it is impossible to create a new Shared Data Block value
consisting of byteLengthDelta bytes, throw a RangeError exception.
g. NOTE: No new Shared Data
Block is constructed and used here. The observable behaviour of growable
SharedArrayBuffers is specified by allocating a max-sized Shared Data Block at
construction time, and this step captures the requirement that implementations that run out of
memory must throw a RangeError.
h. Let readByteLengthRawBytes be AtomicCompareExchangeInSharedBlock(byteLengthBlock,
0, 8, currentByteLengthRawBytes, newByteLengthRawBytes).
i. If ByteListEqual(readByteLengthRawBytes,
currentByteLengthRawBytes) is true, return
undefined.
j. Set currentByteLengthRawBytes to
readByteLengthRawBytes.
Note
Spurious failures of the compare-exchange to update the length are prohibited. If the bounds checking
for the new length passes and the implementation is not out of memory, a ReadModifyWriteSharedMemory
event (i.e. a successful compare-exchange) is always added into the candidate execution.
Parallel calls to SharedArrayBuffer.prototype.grow are totally ordered. For example, consider two
racing calls: sab.grow(10) and sab.grow(20). One of the two calls is
guaranteed to win the race. The call to sab.grow(10) will never shrink sab
even if sab.grow(20) happened first; in that case it will instead throw a RangeError.
25.2.5.4 get SharedArrayBuffer.prototype.growable
SharedArrayBuffer.prototype.growable is an accessor property
whose set accessor function is undefined. Its get accessor function performs the
following steps when called:
25.2.5.5 get SharedArrayBuffer.prototype.maxByteLength
SharedArrayBuffer.prototype.maxByteLength is an accessor property
whose set accessor function is undefined. Its get accessor function performs the
following steps when called:
The initial value of the @@toStringTag property is
the String value "SharedArrayBuffer".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
true }.
25.2.6 Properties of SharedArrayBuffer Instances
SharedArrayBuffer instances inherit properties from the SharedArrayBuffer
prototype object. SharedArrayBuffer instances each have an [[ArrayBufferData]] internal slot. SharedArrayBuffer instances which are not growable
each have an [[ArrayBufferByteLength]] internal slot. SharedArrayBuffer instances
which are growable each have an [[ArrayBufferByteLengthData]] internal slot and an
[[ArrayBufferMaxByteLength]] internal slot.
Note
SharedArrayBuffer instances, unlike ArrayBuffer instances, are never detached.
We recommend that programs be tested in their deployment environments where possible. The amount of
available physical memory differ greatly between hardware devices. Similarly, virtual memory subsystems
also differ greatly between hardware devices as well as operating systems. An application that runs
without out-of-memory errors on a 64-bit desktop web browser could run out of memory on a 32-bit mobile
web browser.
When choosing a value for the "maxByteLength" option for growable
SharedArrayBuffer, we recommend that the smallest possible size for the application
be chosen. We recommend that "maxByteLength" does not exceed 1073741824, or 1GiB.
Please note that successfully constructing a growable
SharedArrayBuffer for a particular maximum size does not guarantee that future grows
will succeed.
Not all loads of a growable
SharedArrayBuffer's length are synchronizing seq-cst loads.
Loads of the length that are for bounds-checking of an integer-indexed
property access, e.g. u8[idx], are not synchronizing. In general, in the absence of
explicit synchronization, one property access being in-bound does not imply a subsequent property access
in the same agent is also in-bound. In contrast,
explicit loads of the length via the length and byteLength getters on
SharedArrayBuffer, %TypedArray%.prototype,
and DataView.prototype are synchronizing. Loads of the length that are performed by built-in methods to
check if a TypedArray is entirely out-of-bounds
are also synchronizing.
We recommend growable
SharedArrayBuffer be implemented as in-place growth via reserving virtual memory up
front.
Because grow operations can happen in parallel with memory accesses on a growable
SharedArrayBuffer, the constraints of the memory model
require that even unordered accesses do not "tear" (bits of their values will not be mixed). In
practice, this means the underlying data block of a growable
SharedArrayBuffer cannot be grown by being copied without stopping the world. We do
not recommend stopping the world as an implementation strategy because it introduces a serialization
point and is slow.
Grown memory must appear zeroed from the moment of its creation, including to any racy accesses in
parallel. This can be accomplished via zero-filled-on-demand virtual memory pages, or careful
synchronization if manually zeroing memory.
Integer-indexed property access on
TypedArray views of growable
SharedArrayBuffers is intended to be optimizable similarly to access on TypedArray views of
non-growable SharedArrayBuffers, because integer-indexed property loads on
are not synchronizing on the underlying buffer's length (see programmer guidelines above). For example,
bounds checks for property accesses may still be hoisted out of loops.
In practice it is difficult to implement growable
SharedArrayBuffer by copying on hosts that do not have virtual memory, such
as those running on embedded devices without an MMU. Memory usage behaviour of growable
SharedArrayBuffers on such hosts may significantly differ from that of
hosts
with virtual memory. Such hosts should clearly communicate memory
usage expectations to users.
25.3 DataView Objects
25.3.1 Abstract Operations For DataView Objects
25.3.1.1 DataView With Buffer Witness Records
A DataView With Buffer Witness
Record is a Record
value used to encapsulate a DataView along with a cached byte length of the viewed buffer. It is used to
help ensure there is a single shared memory read event of the byte length data block when the viewed
buffer is a growable SharedArrayBuffers.
DataView With Buffer Witness Records have the fields listed in Table
72.
The byte length of the object's [[ViewedArrayBuffer]] when the
Record
was created.
25.3.1.2 MakeDataViewWithBufferWitnessRecord ( obj,
order )
The abstract operation MakeDataViewWithBufferWitnessRecord takes arguments obj (a DataView)
and order (seq-cst or unordered) and returns a
DataView With Buffer
Witness Record. It performs the following steps when called:
The abstract operation GetViewByteLength takes argument viewRecord (a DataView With Buffer
Witness Record) and returns a non-negative integer. It performs the following steps
when called:
The abstract operation IsViewOutOfBounds takes argument viewRecord (a DataView With Buffer
Witness Record) and returns a Boolean. It performs the following steps when called:
1. Let view be viewRecord.[[Object]].
2. Let bufferByteLength be viewRecord.[[CachedBufferByteLength]].
3. Assert: IsDetachedBuffer(view.[[ViewedArrayBuffer]]) is true if and only if
bufferByteLength is detached.
4. If bufferByteLength is
detached, return true.
5. Let byteOffsetStart be view.[[ByteOffset]].
6. If view.[[ByteLength]] is
auto, then
a. Let byteOffsetEnd be bufferByteLength.
7. Else,
a. Let byteOffsetEnd be byteOffsetStart +
view.[[ByteLength]].
8. If byteOffsetStart > bufferByteLength or
byteOffsetEnd > bufferByteLength, return true.
9. NOTE: 0-length DataViews are not considered out-of-bounds.
10. Return false.
25.3.1.5 GetViewValue ( view, requestIndex,
isLittleEndian, type )
is the initial value of the "DataView" property of the global
object.
creates and initializes a new DataView when called as a constructor.
is not intended to be called as a function and will throw an exception when called in that manner.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit
the specified DataView behaviour must include a super call to the DataView constructor to create and initialize
subclass instances with the internal state necessary to support the DataView.prototype
built-in methods.
b. If offset + viewByteLength >
bufferByteLength, throw a RangeError exception.
10. Let O be ? OrdinaryCreateFromConstructor(NewTarget,
"%DataView.prototype%", « [[DataView]], [[ViewedArrayBuffer]], [[ByteLength]], [[ByteOffset]] »).
11. If IsDetachedBuffer(buffer)
is true, throw a TypeError exception.
does not have a [[DataView]], [[ViewedArrayBuffer]],
[[ByteLength]], or [[ByteOffset]] internal slot.
25.3.4.1 get DataView.prototype.buffer
DataView.prototype.buffer is an accessor property whose set
accessor function is undefined. Its get accessor function performs the following steps
when called:
3. Assert: O has a [[ViewedArrayBuffer]] internal slot.
4. Let buffer be O.[[ViewedArrayBuffer]].
5. Return buffer.
25.3.4.2 get DataView.prototype.byteLength
DataView.prototype.byteLength is an accessor property whose set
accessor function is undefined. Its get accessor function performs the following steps
when called:
DataView.prototype.byteOffset is an accessor property whose set
accessor function is undefined. Its get accessor function performs the following steps
when called:
The initial value of the @@toStringTag property is
the String value "DataView".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
true }.
25.3.5 Properties of DataView Instances
DataView instances are ordinary objects that inherit
properties from the DataView prototype
object. DataView instances each have [[DataView]], [[ViewedArrayBuffer]], [[ByteLength]], and [[ByteOffset]] internal slots.
Note
The value of the [[DataView]] internal slot is not used within this
specification. The simple presence of that internal slot is used within the specification to identify
objects created using the DataView constructor.
25.4 The Atomics Object
The Atomics object:
is %Atomics%.
is the initial value of the "Atomics" property of the global
object.
does not have a [[Construct]] internal method; it cannot be used as a constructor with the new
operator.
does not have a [[Call]] internal method; it cannot be invoked as a function.
The Atomics object provides functions that operate indivisibly (atomically) on shared memory array cells as
well as functions that let agents wait for and dispatch primitive events.
When used with discipline, the Atomics functions allow multi-agent programs that communicate through shared
memory to execute in a well-understood order even on parallel CPUs. The rules that govern shared-memory
communication are provided by the memory model, defined below.
Note
For informative guidelines for programming and implementing shared memory in ECMAScript, please see the
notes at the end of the memory model section.
25.4.1 Waiter Record
A Waiter Record is a Record value
used to denote a particular call to Atomics.wait or Atomics.waitAsync.
The agent cluster has a store of
WaiterList Records; the store is indexed by (block, i), where block is a
Shared
Data Block and i a
byte offset into the memory of block. WaiterList Records are agent-independent: a lookup
in the store of WaiterList Records by (block, i) will result in the same WaiterList
Record in any agent in the agent cluster.
Each WaiterList Record has a critical section that
controls exclusive access to that WaiterList Record during evaluation. Only a single agent may enter a WaiterList
Record's critical section at one time. Entering and leaving a WaiterList Record's critical section is
controlled by the abstract
operationsEnterCriticalSection and
LeaveCriticalSection.
Operations on a WaiterList Record—adding and removing waiting agents, traversing the list of agents,
suspending and notifying agents on the list, setting and retrieving the
Synchronize event—may
only be performed by agents that have entered the WaiterList
Record's critical section.
The abstract operation RevalidateAtomicAccess takes arguments typedArray (a TypedArray) and
byteIndexInBuffer (an integer) and returns either a normal completion
containingunused or a throw
completion. This operation revalidates the index within the backing buffer for atomic
operations after all argument coercions are performed in Atomics methods, as argument coercions can have
arbitrary side effects, which could cause the buffer to become out of bounds. This operation does not
throw when typedArray's backing buffer is a SharedArrayBuffer. It performs the following steps
when called:
5. If byteIndexInBuffer ≥ taRecord.[[CachedBufferByteLength]], throw a RangeError exception.
6. Return unused.
25.4.3.5 GetWaiterList ( block, i )
The abstract operation GetWaiterList takes arguments block (a Shared
Data Block) and i (a non-negative integer that is evenly
divisible by 4) and returns a WaiterList Record. It
performs the following steps when called:
1. Assert: i and i +
3 are valid byte offsets within the memory of block.
2. Return the WaiterList
Record that is referenced by the pair (block, i).
25.4.3.6 EnterCriticalSection ( WL )
The abstract operation EnterCriticalSection takes argument WL (a WaiterList Record) and
returns unused. It performs the following steps when called:
e. Append enterEvent to eventsRecord.[[EventList]].
f. Append (WL.[[MostRecentLeaveEvent]], enterEvent) to
eventsRecord.[[AgentSynchronizesWith]].
4. Return unused.
EnterCriticalSection has contention when an agent attempting to enter
the critical section must wait
for another agent to leave it. When there is no
contention, FIFO order of EnterCriticalSection calls is observable. When there is contention, an
implementation may choose an arbitrary order but may not cause an agent to wait indefinitely.
25.4.3.7 LeaveCriticalSection ( WL )
The abstract operation LeaveCriticalSection takes argument WL (a WaiterList Record) and
returns unused. It performs the following steps when called:
The abstract operation AddWaiter takes arguments WL (a WaiterList Record) and
waiterRecord (a Waiter Record) and returns
unused. It performs the following steps when called:
2. Assert: There is no Waiter Record in
WL.[[Waiters]] whose [[PromiseCapability]] field is waiterRecord.[[PromiseCapability]] and whose [[AgentSignifier]]
field is waiterRecord.[[AgentSignifier]].
3. Append waiterRecord to WL.[[Waiters]].
4. Return unused.
25.4.3.9 RemoveWaiter ( WL, waiterRecord )
The abstract operation RemoveWaiter takes arguments WL (a WaiterList Record) and
waiterRecord (a Waiter Record) and returns
unused. It performs the following steps when called:
The abstract operation RemoveWaiters takes arguments WL (a WaiterList Record) and
c (a non-negative integer or +∞) and returns a List of
Waiter Records. It performs the
following steps when called:
4. Let L be a List
whose elements are the first n elements of WL.[[Waiters]].
5. Remove the first n elements of WL.[[Waiters]].
6. Return L.
25.4.3.11 SuspendThisAgent ( WL, waiterRecord )
The abstract operation SuspendThisAgent takes arguments WL (a WaiterList Record) and
waiterRecord (a Waiter Record) and returns
unused. It performs the following steps when called:
7. Perform LeaveCriticalSection(WL)
and suspend the surrounding agent until the
time is waiterRecord.[[TimeoutTime]], performing the combined
operation in such a way that a notification that arrives after the critical section is
exited but before the suspension takes effect is not lost. The surrounding
agent can only wake from suspension due to a timeout or due to another agent
calling NotifyWaiter with arguments
WL and thisAgent (i.e. via a call to Atomics.notify).
The abstract operation NotifyWaiter takes arguments WL (a WaiterList Record) and
waiterRecord (a Waiter Record) and returns
unused. It performs the following steps when called:
The abstract operation EnqueueResolveInAgentJob takes arguments agentSignifier (an agent
signifier), promiseCapability (a PromiseCapability
Record), and resolution (an ECMAScript
language value) and returns unused. It performs the following
steps when called:
1. Let resolveJob be a new JobAbstract Closure with no
parameters that captures agentSignifier, promiseCapability, and
resolution and performs the following steps when called:
a. NOTE: There is no special handling of synchronous immediate
timeouts. Asynchronous immediate timeouts have special handling in order to fail fast and avoid
unnecessary Promise jobs.
25. Let timeoutTime be ℝ(now) +
t + additionalTimeout.
26. NOTE: When t is +∞, timeoutTime is also
+∞.
27. Let waiterRecord be a new Waiter Record { [[AgentSignifier]]: thisAgent, [[PromiseCapability]]: promiseCapability, [[TimeoutTime]]: timeoutTime, [[Result]]:
"ok" }.
additionalTimeout allows implementations to pad timeouts as necessary, such as for
reducing power consumption or coarsening timer resolution to mitigate timing attacks. This value may
differ from call to call of DoWait.
The abstract operation EnqueueAtomicsWaitAsyncTimeoutJob takes arguments WL (a WaiterList Record) and
waiterRecord (a Waiter Record) and returns
unused. It performs the following steps when called:
1. Let timeoutJob be a new JobAbstract Closure with no
parameters that captures WL and waiterRecord and performs the following steps
when called:
The abstract operation AtomicCompareExchangeInSharedBlock takes arguments block (a Shared
Data Block), byteIndexInBuffer (an integer),
elementSize (a non-negative integer), expectedBytes (a
List of
byte
values), and
replacementBytes (a List of
byte
values) and returns a
List of
byte
values. It performs the
following steps when called:
3. Let rawBytesRead be a List of
length elementSize whose elements are nondeterministically chosen byte
values.
4. NOTE: In implementations, rawBytesRead is the result
of a load-link, of a load-exclusive, or of an operand of a read-modify-write instruction on the
underlying hardware. The nondeterminism is a semantic prescription of the memory
model to describe
observable behaviour of hardware with weak consistency.
5. NOTE: The comparison of the expected value and the read value is
performed outside of the read-modify-write modification
function to avoid needlessly strong synchronization when the expected value is not
equal to the read value.
6. If ByteListEqual(rawBytesRead,
expectedBytes) is true, then
a. Let second be a new read-modify-write modification
function with parameters (oldBytes, newBytes) that
captures nothing and performs the following steps atomically when called:
i. Return newBytes.
b. Let event be ReadModifyWriteSharedMemory
{ [[Order]]: seq-cst, [[NoTear]]: true, [[Block]]:
block, [[ByteIndex]]: byteIndexInBuffer, [[ElementSize]]: elementSize, [[Payload]]: replacementBytes, [[ModifyOp]]: second }.
7. Else,
a. Let event be ReadSharedMemory
{ [[Order]]: seq-cst, [[NoTear]]: true, [[Block]]:
block, [[ByteIndex]]: byteIndexInBuffer, [[ElementSize]]: elementSize }.
8. Append event to eventsRecord.[[EventList]].
9. Append Chosen Value
Record { [[Event]]: event, [[ChosenValue]]: rawBytesRead } to execution.[[ChosenValues]].
10. Return rawBytesRead.
25.4.3.17 AtomicReadModifyWrite ( typedArray, index,
value, op )
The abstract operation ByteListBitwiseOp takes arguments op (&,
^, or |), xBytes (a List of
byte
values), and
yBytes (a List of
byte
values) and returns a
List of
byte
values. The operation
atomically performs a bitwise operation on all byte values of the arguments and
returns a List of
byte
values. It performs the
following steps when called:
1. Assert: xBytes and
yBytes have the same number of elements.
ii. Let resultByte be the result of applying the
bitwise inclusive OR operation to xByte and yByte.
e. Set i to i + 1.
f. Append resultByte to result.
5. Return result.
25.4.3.19 ByteListEqual ( xBytes, yBytes )
The abstract operation ByteListEqual takes arguments xBytes (a List of
byte
values) and yBytes
(a List of
byte
values) and returns a
Boolean. It performs the following steps when called:
1. If xBytes and yBytes do not have the same
number of elements, return false.
2. Let i be 0.
3. For each element xByte of xBytes, do
a. Let yByte be yBytes[i].
b. If xByte ≠ yByte, return
false.
c. Set i to i + 1.
4. Return true.
25.4.4 Atomics.add ( typedArray, index, value
)
This function performs the following steps when called:
3. Let add be a new read-modify-write modification
function with parameters (xBytes, yBytes) that captures
type and isLittleEndian and performs the following steps atomically when called:
This function performs the following steps when called:
1. Let and be a new read-modify-write modification
function with parameters (xBytes, yBytes) that captures nothing
and performs the following steps atomically when called:
25.4.7 Atomics.exchange ( typedArray, index,
value )
This function performs the following steps when called:
1. Let second be a new read-modify-write modification
function with parameters (oldBytes, newBytes) that captures
nothing and performs the following steps atomically when called:
This function is an optimization primitive. The intuition is that if the atomic step of an atomic
primitive (compareExchange, load, store, add,
sub, and, or, xor, or exchange) on a
datum of size n bytes will be performed without the surrounding
agent acquiring a lock outside the n bytes comprising the datum, then
Atomics.isLockFree(n) will return true. High-performance
algorithms will use this function to determine whether to use locks or atomic operations in critical sections. If an
atomic primitive is not lock-free then it is often more efficient for an algorithm to provide its own
locking.
Atomics.isLockFree(4) always returns true as that can be supported on
all known relevant hardware. Being able to assume this will generally simplify programs.
Regardless of the value returned by this function, all atomic operations are guaranteed to be atomic.
For example, they will never have a visible operation take place in the middle of the operation (e.g.,
"tearing").
25.4.9 Atomics.load ( typedArray, index )
This function performs the following steps when called:
This function performs the following steps when called:
1. Let or be a new read-modify-write modification
function with parameters (xBytes, yBytes) that captures nothing
and performs the following steps atomically when called:
3. Let subtract be a new read-modify-write modification
function with parameters (xBytes, yBytes) that captures
type and isLittleEndian and performs the following steps atomically when called:
This function puts the surrounding agent in a wait queue
and suspends it until notified or until the wait times out, returning a String differentiating those cases.
This function performs the following steps when called:
1. Let xor be a new read-modify-write modification
function with parameters (xBytes, yBytes) that captures nothing
and performs the following steps atomically when called:
does not have a [[Construct]] internal method; it cannot be used as a constructor with the new
operator.
does not have a [[Call]] internal method; it cannot be invoked as a function.
The JSON Data Interchange Format is defined in ECMA-404. The JSON interchange format used in this
specification is exactly that described by ECMA-404. Conforming implementations of JSON.parse and
JSON.stringify must support the exact interchange format described in the ECMA-404 specification
without any deletions or extensions to the format.
25.5.1 JSON.parse ( text [ , reviver ] )
This function parses a JSON text (a JSON-formatted String) and produces an ECMAScript language
value. The JSON format represents literals, arrays, and objects with a syntax similar to
the syntax for ECMAScript literals, Array Initializers, and Object Initializers. After parsing, JSON objects
are realized as ECMAScript objects. JSON arrays are realized as ECMAScript Array instances. JSON strings,
numbers, booleans, and null are realized as ECMAScript Strings, Numbers, Booleans, and
null.
The optional reviver parameter is a function that takes two parameters, key and
value. It can filter and transform the results. It is called with each of the
key/value pairs produced by the parse, and its return value is used instead of the
original value. If it returns what it received, the structure is not modified. If it returns
undefined then the property is deleted from the result.
2. Parse StringToCodePoints(jsonString)
as a JSON text as specified in ECMA-404. Throw a SyntaxError exception if it is not a
valid JSON text as defined in that specification.
Valid JSON text is a subset of the ECMAScript PrimaryExpression syntax.
Step 2 verifies that
jsonString conforms to that subset, and step 10
asserts that that parsing and evaluation returns a value of an appropriate type.
However, because 13.2.5.5
behaves differently during JSON.parse, the same source text can produce different results
when evaluated as a PrimaryExpression rather
than as JSON. Furthermore, the Early Error for duplicate "__proto__" properties in
object literals, which likewise does not apply during JSON.parse, means that not all texts
accepted by JSON.parse are valid as a PrimaryExpression, despite
matching the grammar.
It is not permitted for a conforming implementation of JSON.parse to extend the JSON
grammars. If an implementation wishes to support a modified or extended JSON interchange format it must do
so by defining a different parse function.
Note 2
In the case where there are duplicate name Strings within an object, lexically preceding values for
the same key shall be overwritten.
25.5.2 JSON.stringify ( value [ , replacer [ ,
space ] ] )
This function returns a String in UTF-16 encoded JSON format representing an ECMAScript language
value, or undefined. It can take three parameters. The
value parameter is an ECMAScript language
value, which is usually an object or array, although it can also be a String, Boolean,
Number or null. The optional replacer parameter is either a function that
alters the way objects and arrays are stringified, or an array of Strings and Numbers that acts as an
inclusion list for selecting the object properties that will be stringified. The optional space
parameter is a
String or Number that allows the result to have white space injected into it to improve
human readability.
12. Let state be the JSON Serialization
Record { [[ReplacerFunction]]: ReplacerFunction,
[[Stack]]: stack, [[Indent]]:
indent, [[Gap]]: gap, [[PropertyList]]: PropertyList }.
JSON structures are allowed to be nested to any depth, but they must be acyclic. If value is
or contains a cyclic structure, then this function must throw a TypeError exception.
This is an example of a value that cannot be stringified:
a = [];
a[0] = a;
my_text = JSON.stringify(a); // This must throw a TypeError.
Note 2
Symbolic primitive values are rendered as follows:
The null value is rendered in JSON text as the String value
"null".
The undefined value is not rendered.
The true value is rendered in JSON text as the String value
"true".
The false value is rendered in JSON text as the String value
"false".
Note 3
String values are wrapped in QUOTATION MARK (") code units. The code units "
and \ are escaped with \ prefixes. Control characters code units are replaced
with escape sequences \uHHHH, or with the shorter forms, \b (BACKSPACE),
\f (FORM FEED), \n (LINE FEED), \r (CARRIAGE RETURN),
\t (CHARACTER TABULATION).
Note 4
Finite numbers are stringified as if by
calling ToString(number).
NaN and Infinity regardless of sign are represented as the String
value "null".
Note 5
Values that do not have a JSON representation (such as undefined and functions) do
not produce a String. Instead they produce the undefined value. In arrays these
values are represented as the String value "null". In objects an unrepresentable
value causes the property to be excluded from stringification.
Note 6
An object is rendered as U+007B (LEFT CURLY BRACKET) followed by zero or more properties, separated
with a U+002C (COMMA), closed with a U+007D (RIGHT CURLY BRACKET). A property is a quoted String
representing the property name, a U+003A (COLON),
and then the stringified property value. An array is rendered as an opening U+005B (LEFT SQUARE BRACKET)
followed by zero or more values, separated with a U+002C (COMMA), closed with a U+005D (RIGHT SQUARE
BRACKET).
25.5.2.1 JSON Serialization Record
A JSON Serialization Record is a Record
value used to enable serialization to the JSON format.
JSON Serialization Records have the fields listed in Table
75.
The abstract operation SerializeJSONProperty takes arguments state (a JSON Serialization
Record), key (a String), and holder (an Object) and returns
either a normal completion
containing either a String or undefined, or a throw
completion. It performs the following steps when called:
The abstract operation QuoteJSONString takes argument value (a String) and returns a String.
It wraps value in 0x0022 (QUOTATION MARK) code units and escapes certain other code units
within it. This operation interprets value as a sequence of UTF-16 encoded code points, as
described in 6.1.4.
It performs the following steps when called:
1. Let product be the String value consisting solely of
the code unit 0x0022 (QUOTATION MARK).
a. If C is listed in the “Code Point” column of
Table
76, then
i. Set product to the string-concatenation
of product and the escape sequence for C as specified in the “Escape
Sequence” column of the corresponding row.
b. Else if C has a numeric value less than 0x0020
(SPACE) or C has the same numeric value as a leading
surrogate or trailing surrogate,
then
i. Let unit be the code unit whose numeric value
is the numeric value of C.
3. Set product to the string-concatenation of
product and the code unit 0x0022 (QUOTATION MARK).
4. Return product.
Table 76: JSON Single Character Escape Sequences
Code Point
Unicode Character Name
Escape Sequence
U+0008
BACKSPACE
\b
U+0009
CHARACTER TABULATION
\t
U+000A
LINE FEED (LF)
\n
U+000C
FORM FEED (FF)
\f
U+000D
CARRIAGE RETURN (CR)
\r
U+0022
QUOTATION MARK
\"
U+005C
REVERSE SOLIDUS
\\
25.5.2.4 UnicodeEscape ( C )
The abstract operation UnicodeEscape takes argument C (a code unit) and returns a String. It
represents C as a Unicode escape sequence. It performs the following steps when called:
i. Let properties be the String value formed by
concatenating all the element Strings of partial with each adjacent pair of Strings
separated with the code unit 0x002C (COMMA). A comma is not inserted either before the first
String or after the last String.
i. Let separator be the string-concatenation
of the code unit 0x002C (COMMA), the code unit 0x000A (LINE FEED), and state.[[Indent]].
ii. Let properties be the String value formed by
concatenating all the element Strings of partial with each adjacent pair of Strings
separated with separator. The separator String is not inserted either
before the first String or after the last String.
iii. Let final be the string-concatenation
of "{", the code unit 0x000A (LINE FEED), state.[[Indent]], properties, the code unit 0x000A (LINE FEED),
stepback, and "}".
i. Let properties be the String value formed by
concatenating all the element Strings of partial with each adjacent pair of Strings
separated with the code unit 0x002C (COMMA). A comma is not inserted either before the first
String or after the last String.
i. Let separator be the string-concatenation
of the code unit 0x002C (COMMA), the code unit 0x000A (LINE FEED), and state.[[Indent]].
ii. Let properties be the String value formed by
concatenating all the element Strings of partial with each adjacent pair of Strings
separated with separator. The separator String is not inserted either
before the first String or after the last String.
iii. Let final be the string-concatenation
of "[", the code unit 0x000A (LINE FEED), state.[[Indent]], properties, the code unit 0x000A (LINE FEED),
stepback, and "]".
11. Remove the last element of state.[[Stack]].
12. Set state.[[Indent]] to
stepback.
13. Return final.
Note
The representation of arrays includes only the elements in the interval from
+0𝔽 (inclusive) to array.length (exclusive). Properties
whose keys are not array indices are excluded from
the stringification. An array is stringified as an opening LEFT SQUARE BRACKET, elements separated by
COMMA, and a closing RIGHT SQUARE BRACKET.
25.5.3 JSON [ @@toStringTag ]
The initial value of the @@toStringTag property is the
String value "JSON".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
true }.
26 Managing Memory
26.1 WeakRef Objects
A WeakRef is an object that is
used to refer to a target object or symbol without preserving it from garbage collection. WeakRefs can be dereferenced
to allow access to the target value, if the target hasn't been reclaimed by garbage collection.
is the initial value of the "WeakRef" property of the global
object.
creates and initializes a new WeakRef when called as a constructor.
is not intended to be called as a function and will throw an exception when called in that manner.
may be used as the value in an extends clause of a class definition. Subclass constructors that intend to inherit
the specified WeakRef behaviour must include a super call to the
WeakRefconstructor to create and initialize
the subclass instance with the internal state necessary to support the WeakRef.prototype
built-in methods.
26.1.1.1 WeakRef ( target )
This function performs the following steps when called:
1. If NewTarget is undefined, throw a
TypeError exception.
2. If CanBeHeldWeakly(target)
is false, throw a TypeError exception.
If the WeakRef returns a
target value that is not undefined, then this target value
should not be garbage collected until the current execution of ECMAScript code has completed. The
AddToKeptObjects
operation makes sure read consistency is maintained.
let target = { foo() {} };
let weakRef = newWeakRef(target);
// ... later ...if (weakRef.deref()) {
weakRef.deref().foo();
}
In the above example, if the first deref does not evaluate to undefined then the
second deref cannot either.
26.1.3.3 WeakRef.prototype [ @@toStringTag ]
The initial value of the @@toStringTag property is
the String value "WeakRef".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
true }.
26.1.4 WeakRef Abstract Operations
26.1.4.1 WeakRefDeref ( weakRef )
The abstract operation WeakRefDeref takes argument weakRef (a WeakRef) and returns an
ECMAScript language
value. It performs the following steps when called:
A FinalizationRegistry
is an object that manages registration and unregistration of cleanup operations that are performed when target
objects and symbols are garbage collected.
is the initial value of the "FinalizationRegistry" property of the global
object.
creates and initializes a new FinalizationRegistry when called as a constructor.
is not intended to be called as a function and will throw an exception when called in that manner.
may be used as the value in an extends clause of a class definition. Subclass constructors that intend to inherit
the specified FinalizationRegistry behaviour must include a super call to the
FinalizationRegistryconstructor to create and initialize
the subclass instance with the internal state necessary to support the
FinalizationRegistry.prototype built-in methods.
26.2.1.1 FinalizationRegistry ( cleanupCallback )
This function performs the following steps when called:
1. If NewTarget is undefined, throw a
TypeError exception.
2. If IsCallable(cleanupCallback)
is false, throw a TypeError exception.
3. Let finalizationRegistry be ? OrdinaryCreateFromConstructor(NewTarget,
"%FinalizationRegistry.prototype%", « [[Realm]], [[CleanupCallback]], [[Cells]] »).
a. If unregisterToken is not
undefined, throw a TypeError exception.
b. Set unregisterToken to
empty.
6. Let cell be the Record
{ [[WeakRefTarget]]: target, [[HeldValue]]: heldValue, [[UnregisterToken]]: unregisterToken }.
7. Append cell to finalizationRegistry.[[Cells]].
8. Return undefined.
Note
Based on the algorithms and definitions in this specification, cell.[[HeldValue]] is live when
finalizationRegistry.[[Cells]] contains cell; however,
this does not necessarily mean that cell.[[UnregisterToken]] or
cell.[[Target]] are live. For
example, registering an object with itself as its unregister token would not keep the object alive
forever.
An interface is a set of property keys whose associated
values match a specific specification. Any object that provides all the properties as described by an
interface's specification conforms to that interface. An interface is not represented by a distinct
object. There may be many separately implemented objects that conform to any interface. An individual object
may conform to multiple interfaces.
27.1.1.1 The Iterable Interface
The Iterable interface includes the property described in Table
77:
Table 77: Iterable Interface Required Properties
Property
Value
Requirements
@@iterator
a function that returns an Iterator object
The returned object must conform to the Iterator interface.
27.1.1.2 The Iterator Interface
An object that implements the Iterator interface must include the property in Table
78. Such objects may also implement the properties in Table
79.
Table 78: Iterator Interface Required Properties
Property
Value
Requirements
"next"
a function that returns an IteratorResult object
The returned object must conform to the IteratorResult interface. If a previous call to
the next method of an Iterator has returned an IteratorResult object
whose "done" property is true, then all subsequent calls
to the next method of that object should also return an IteratorResult
object whose "done" property is true. However, this
requirement is not enforced.
Note 1
Arguments may be passed to the next function but their interpretation and validity is
dependent upon the target Iterator. The for-of statement and other common users of
Iterators do not pass any arguments, so Iterator objects that expect to be used in
such a manner must be prepared to deal with being called with no arguments.
Table 79: Iterator Interface Optional Properties
Property
Value
Requirements
"return"
a function that returns an IteratorResult object
The returned object must conform to the IteratorResult interface. Invoking this method
notifies the Iterator object that the caller does not intend to make any more
next method calls to the Iterator. The returned IteratorResult object
will typically have a "done" property whose value is true,
and a "value" property with the value passed as the argument of the
return method. However, this requirement is not enforced.
"throw"
a function that returns an IteratorResult object
The returned object must conform to the IteratorResult interface. Invoking this method
notifies the Iterator object that the caller has detected an error condition. The
argument may be used to identify the error condition and typically will be an exception object.
A typical response is to throw the value passed as the argument. If the method does
not throw, the returned IteratorResult object will typically have a
"done" property whose value is true.
Note 2
Typically callers of these methods should check for their existence before invoking them. Certain
ECMAScript language features including for-of, yield*, and
array destructuring call these methods after performing an existence check. Most ECMAScript library
functions that accept Iterable objects as arguments also conditionally call them.
27.1.1.3 The AsyncIterable Interface
The AsyncIterable interface includes the properties described in Table 80:
The returned object must conform to the AsyncIterator interface.
27.1.1.4 The AsyncIterator Interface
An object that implements the AsyncIterator interface must include the properties in Table 81. Such
objects may also implement the properties in Table
82.
a function that returns a promise for an IteratorResult object
The returned promise, when fulfilled, must fulfill with an object that conforms to the
IteratorResult interface. If a previous call to the next method of an
AsyncIterator has returned a promise for an IteratorResult object whose
"done" property is true, then all subsequent calls to
the next method of that object should also return a promise for an
IteratorResult object whose "done" property is
true. However, this requirement is not enforced.
Additionally, the IteratorResult object that serves as a fulfillment value should have
a "value" property whose value is not a promise (or "thenable"). However,
this requirement is also not enforced.
Note 1
Arguments may be passed to the next function but their interpretation and validity is
dependent upon the target AsyncIterator. The
for-await-of statement and other common users of
AsyncIterators do not pass any arguments, so AsyncIterator objects that expect to be
used in such a manner must be prepared to deal with being called with no arguments.
a function that returns a promise for an IteratorResult object
The returned promise, when fulfilled, must fulfill with an object that conforms to the
IteratorResult interface. Invoking this method notifies the AsyncIterator object
that the caller does not intend to make any more next method calls to the
AsyncIterator. The returned promise will fulfill with an IteratorResult object
which will typically have a "done" property whose value is
true, and a "value" property with the value passed as
the argument of the return method. However, this requirement is not enforced.
Additionally, the IteratorResult object that serves as a fulfillment value should have
a "value" property whose value is not a promise (or "thenable"). If the
argument value is used in the typical manner, then if it is a rejected promise, a promise
rejected with the same reason should be returned; if it is a fulfilled promise, then its
fulfillment value should be used as the "value" property of the returned
promise's IteratorResult object fulfillment value. However, these requirements are also
not enforced.
"throw"
a function that returns a promise for an IteratorResult object
The returned promise, when fulfilled, must fulfill with an object that conforms to the
IteratorResult interface. Invoking this method notifies the AsyncIterator object
that the caller has detected an error condition. The argument may be used to identify the
error condition and typically will be an exception object. A typical response is to return a
rejected promise which rejects with the value passed as the argument.
If the returned promise is fulfilled, the IteratorResult fulfillment value will
typically have a "done" property whose value is true.
Additionally, it should have a "value" property whose value is not a
promise (or "thenable"), but this requirement is not enforced.
Note 2
Typically callers of these methods should check for their existence before invoking them. Certain
ECMAScript language features including for-await-of and
yield* call these methods after performing an existence check.
27.1.1.5 The IteratorResult Interface
The IteratorResult interface includes the properties listed in Table
83:
Table 83: IteratorResult Interface Properties
Property
Value
Requirements
"done"
a Boolean
This is the result status of an iteratornext method call. If the end of
the iterator was reached "done" is true. If the end was
not reached "done" is false and a value is available. If a
"done" property (either own or inherited) does not exist, it is considered to
have the value false.
If done is false, this is the current iteration element value. If done is
true, this is the return value of the iterator, if it supplied one. If the
iterator does not have a return value, "value" is
undefined. In that case, the "value" property may be
absent from the conforming object if it does not inherit an explicit "value"
property.
All objects defined in this specification that implement the Iterator interface also inherit from
%IteratorPrototype%. ECMAScript code may also define objects that inherit from %IteratorPrototype%. The
%IteratorPrototype% object provides a place where additional methods that are applicable to all iterator
objects may be added.
The following expression is one way that ECMAScript code can access the %IteratorPrototype% object:
All objects defined in this specification that implement the AsyncIterator interface also inherit from
%AsyncIteratorPrototype%. ECMAScript code may also define objects that inherit from
%AsyncIteratorPrototype%. The %AsyncIteratorPrototype% object provides a place where additional methods
that are applicable to all async iterator objects may be added.
This function performs the following steps when called:
1. Return the this value.
The value of the "name" property of this function is
"[Symbol.asyncIterator]".
27.1.4 Async-from-Sync Iterator Objects
An Async-from-Sync Iterator object is an async iterator that adapts a specific synchronous iterator. There
is not a named constructor for Async-from-Sync Iterator
objects. Instead, Async-from-Sync iterator objects are created by the CreateAsyncFromSyncIterator
abstract operation as needed.
The abstract operation CreateAsyncFromSyncIterator takes argument syncIteratorRecord (an
Iterator Record) and returns
an Iterator Record. It is used
to create an async Iterator Record from a
synchronous Iterator Record. It performs
the following steps when called:
27.1.4.3 Properties of Async-from-Sync Iterator Instances
Async-from-Sync Iterator instances are ordinary objects that inherit
properties from the %AsyncFromSyncIteratorPrototype%
intrinsic object. Async-from-Sync Iterator instances are initially created with the internal slots listed
in Table
84. Async-from-Sync Iterator instances are not directly observable from ECMAScript
code.
Table 84: Internal Slots of Async-from-Sync Iterator Instances
The abstract operation AsyncFromSyncIteratorContinuation takes arguments result (an Object)
and promiseCapability (a PromiseCapability
Record for an intrinsic %Promise%) and returns a
Promise. It performs the following steps when called:
1. NOTE: Because promiseCapability is derived from the
intrinsic %Promise%, the calls
to promiseCapability.[[Reject]] entailed by the use IfAbruptRejectPromise
below are guaranteed not to throw.
10. NOTE: onFulfilled is used when processing the
"value" property of an IteratorResult object in order to wait for its value if it
is a promise and re-package the result in a new "unwrapped" IteratorResult object.
A Promise is an object that is used as a placeholder for the eventual results of a deferred (and possibly
asynchronous) computation.
Any Promise is in one of three mutually exclusive states: fulfilled, rejected, and
pending:
A promise p is fulfilled if p.then(f, r) will immediately enqueue a Job to call the
function f.
A promise p is rejected if p.then(f, r) will immediately enqueue a Job to call the
function r.
A promise is pending if it is neither fulfilled nor rejected.
A promise is said to be settled if it is not pending, i.e. if it is either fulfilled or rejected.
A promise is resolved if it is settled or if it has been “locked in” to match the state of another
promise. Attempting to resolve or reject a resolved promise has no effect. A promise is unresolved if
it is not resolved. An unresolved promise is always in the pending state. A resolved promise may be pending,
fulfilled or rejected.
27.2.1 Promise Abstract Operations
27.2.1.1 PromiseCapability Records
A PromiseCapability Record is a Record
value used to encapsulate a Promise or promise-like object along with the functions that are capable of
resolving or rejecting that promise. PromiseCapability Records are produced by the NewPromiseCapability
abstract operation.
PromiseCapability Records have the fields listed in Table 85.
a. Perform ? Call(capability.[[Reject]], undefined, « value.[[Value]] »).
b. Return capability.[[Promise]].
3. Else,
a. Set value to ! value.
27.2.1.2 PromiseReaction Records
A PromiseReaction Record is a Record
value used to store information about how a promise should react when it becomes resolved or rejected with
a given value. PromiseReaction Records are created by the PerformPromiseThen abstract
operation, and are used by the Abstract Closure returned by
NewPromiseReactionJob.
PromiseReaction Records have the fields listed in Table
86.
The function that should be applied to the incoming value, and whose return value will govern
what happens to the derived promise. If [[Handler]] is
empty, a function that depends on the value of [[Type]] will be used instead.
27.2.1.3 CreateResolvingFunctions ( promise )
The abstract operation CreateResolvingFunctions takes argument promise (a Promise) and returns
a Record with
fields [[Resolve]] (a function object) and [[Reject]] (a function object). It performs the
following steps when called:
1. Let alreadyResolved be the Record
{ [[Value]]: false }.
The "length" property of a promise resolve function is
1𝔽.
27.2.1.4 FulfillPromise ( promise, value )
The abstract operation FulfillPromise takes arguments promise (a Promise) and value
(an ECMAScript language
value) and returns unused. It performs the following steps when
called:
1. Assert: The value of
promise.[[PromiseState]] is pending.
2. Let reactions be promise.[[PromiseFulfillReactions]].
3. Set promise.[[PromiseResult]]
to value.
4. Set promise.[[PromiseFulfillReactions]] to undefined.
5. Set promise.[[PromiseRejectReactions]] to undefined.
1. If IsConstructor(C)
is false, throw a TypeError exception.
2. NOTE: C is assumed to be a constructor function that supports
the parameter conventions of the Promise constructor (see 27.2.3.1).
3. Let resolvingFunctions be the Record
{ [[Resolve]]: undefined, [[Reject]]: undefined }.
4. Let executorClosure be a new Abstract Closure with
parameters (resolve, reject) that captures resolvingFunctions and
performs the following steps when called:
a. If resolvingFunctions.[[Resolve]] is not undefined, throw a
TypeError exception.
b. If resolvingFunctions.[[Reject]] is not undefined, throw a
TypeError exception.
7. If IsCallable(resolvingFunctions.[[Resolve]]) is false, throw a TypeError
exception.
8. If IsCallable(resolvingFunctions.[[Reject]]) is false, throw a TypeError
exception.
9. Return the PromiseCapability
Record { [[Promise]]: promise, [[Resolve]]: resolvingFunctions.[[Resolve]],
[[Reject]]: resolvingFunctions.[[Reject]] }.
Note
This abstract operation supports Promise subclassing, as it is generic on any constructor that calls a passed
executor function argument in the same way as the Promise constructor. It
is used to generalize static methods of the Promise constructor to any subclass.
27.2.1.6 IsPromise ( x )
The abstract operation IsPromise takes argument x (an ECMAScript language
value) and returns a Boolean. It checks for the promise brand on an object. It performs
the following steps when called:
2. If x does not have a [[PromiseState]] internal slot, return false.
3. Return true.
27.2.1.7 RejectPromise ( promise, reason )
The abstract operation RejectPromise takes arguments promise (a Promise) and reason
(an ECMAScript language
value) and returns unused. It performs the following steps when
called:
1. Assert: The value of
promise.[[PromiseState]] is pending.
2. Let reactions be promise.[[PromiseRejectReactions]].
3. Set promise.[[PromiseResult]]
to reason.
4. Set promise.[[PromiseFulfillReactions]] to undefined.
5. Set promise.[[PromiseRejectReactions]] to undefined.
The abstract operation TriggerPromiseReactions takes arguments reactions (a List of
PromiseReaction
Records) and argument (an ECMAScript
language value) and returns unused. It enqueues a new Job for each
record in reactions. Each such Job processes the [[Type]] and [[Handler]] of the PromiseReaction
Record, and if the [[Handler]] is not
empty, calls it passing the given argument. If the [[Handler]] is empty, the behaviour is determined by the [[Type]]. It performs the following steps when called:
The host-defined abstract operation
HostPromiseRejectionTracker takes arguments promise (a Promise) and operation
("reject" or "handle") and returns unused. It
allows host environments to track
promise rejections.
The default implementation of HostPromiseRejectionTracker is to return unused.
Note 1
HostPromiseRejectionTracker is called in two scenarios:
When a promise is rejected without any handlers, it is called with its operation
argument set to "reject".
When a handler is added to a rejected promise for the first time, it is called with its
operation argument set to "handle".
A typical implementation of HostPromiseRejectionTracker might try to notify developers of unhandled
rejections, while also being careful to notify them if such previous notifications are later
invalidated by new handlers being attached.
Note 2
If operation is "handle", an implementation should not hold a reference
to promise in a way that would interfere with garbage collection. An implementation may
hold a reference to promise if operation is "reject", since
it is expected that rejections will be rare and not on hot code paths.
The abstract operation NewPromiseReactionJob takes arguments reaction (a PromiseReaction
Record) and argument (an ECMAScript
language value) and returns a Record with
fields [[Job]] (a JobAbstract
Closure) and [[Realm]] (a Realm Record or
null). It returns a new JobAbstract
Closure that applies the appropriate handler to the incoming value, and uses the
handler's return value to resolve or reject the derived promise associated with that handler. It performs
the following steps when called:
1. Let job be a new JobAbstract Closure with no
parameters that captures reaction and argument and performs the following steps
when called:
a. Let promiseCapability be reaction.[[Capability]].
d. NOTE: handlerRealm is never
null unless the handler is undefined. When the handler is a
revoked Proxy and no ECMAScript code runs, handlerRealm is used to create error
objects.
4. Return the Record
{ [[Job]]: job, [[Realm]]:
handlerRealm }.
27.2.2.2 NewPromiseResolveThenableJob ( promiseToResolve,
thenable, then )
The abstract operation NewPromiseResolveThenableJob takes arguments promiseToResolve (a
Promise), thenable (an Object), and then (a JobCallback Record) and
returns a Record with
fields [[Job]] (a JobAbstract
Closure) and [[Realm]] (a Realm Record). It
performs the following steps when called:
1. Let job be a new JobAbstract Closure with no
parameters that captures promiseToResolve, thenable, and then and
performs the following steps when called:
5. NOTE: thenRealm is never null. When
then.[[Callback]] is a revoked Proxy and no code runs,
thenRealm is used to create error objects.
6. Return the Record
{ [[Job]]: job, [[Realm]]:
thenRealm }.
Note
This Job
uses the supplied thenable and its
then method to resolve the given promise. This process must take place as a Job to ensure
that the evaluation of the then method occurs after evaluation of any surrounding code
has completed.
is the initial value of the "Promise" property of the global
object.
creates and initializes a new Promise when called as a constructor.
is not intended to be called as a function and will throw an exception when called in that manner.
may be used as the value in an extends clause of a class definition. Subclass constructors that intend to inherit
the specified Promise behaviour must include a super call to the Promise constructor to create and initialize
the subclass instance with the internal state necessary to support the Promise and
Promise.prototype built-in methods.
27.2.3.1 Promise ( executor )
This function performs the following steps when called:
1. If NewTarget is undefined, throw a
TypeError exception.
2. If IsCallable(executor)
is false, throw a TypeError exception.
3. Let promise be ? OrdinaryCreateFromConstructor(NewTarget,
"%Promise.prototype%", « [[PromiseState]], [[PromiseResult]], [[PromiseFulfillReactions]], [[PromiseRejectReactions]], [[PromiseIsHandled]] »).
4. Set promise.[[PromiseState]]
to pending.
5. Set promise.[[PromiseFulfillReactions]] to a new empty List.
6. Set promise.[[PromiseRejectReactions]] to a new empty List.
a. Perform ? Call(resolvingFunctions.[[Reject]], undefined, « completion.[[Value]] »).
11. Return promise.
Note
The executor argument must be a function object. It is called
for initiating and reporting completion of the possibly deferred action represented by this Promise.
The executor is called with two arguments: resolve and reject. These are
functions that may be used by the executor function to report eventual completion or
failure of the deferred computation. Returning from the executor function does not mean that the
deferred action has been completed but only that the request to eventually perform the deferred action
has been accepted.
The resolve function that is passed to an executor function accepts a single
argument. The executor code may eventually call the resolve function to indicate
that it wishes to resolve the associated Promise. The argument passed to the resolve
function represents the eventual value of the deferred action and can be either the actual fulfillment
value or another promise which will provide the value if it is fulfilled.
The reject function that is passed to an executor function accepts a single
argument. The executor code may eventually call the reject function to indicate
that the associated Promise is rejected and will never be fulfilled. The argument passed to the
reject function is used as the rejection value of the promise. Typically it will be an
Error object.
The resolve and reject functions passed to an executor function by the Promise constructor have the capability to
actually resolve and reject the associated promise. Subclasses may have different constructor behaviour that passes
in customized values for resolve and reject.
This function returns a new promise which is fulfilled with an array of fulfillment values for the passed
promises, or rejects with the reason of the first passed promise that rejects. It resolves all elements of
the passed iterable to promises as it runs this algorithm.
g. Let onFulfilled be CreateBuiltinFunction(steps,
length, "", « [[AlreadyCalled]], [[Index]], [[Values]], [[Capability]], [[RemainingElements]] »).
h. Set onFulfilled.[[AlreadyCalled]] to false.
i. Set onFulfilled.[[Index]] to index.
j. Set onFulfilled.[[Values]] to values.
k. Set onFulfilled.[[Capability]] to resultCapability.
l. Set onFulfilled.[[RemainingElements]] to remainingElementsCount.
m. Set remainingElementsCount.[[Value]] to remainingElementsCount.[[Value]] + 1.
n. Perform ? Invoke(nextPromise, "then",
« onFulfilled, resultCapability.[[Reject]] »).
o. Set index to index + 1.
27.2.4.1.3Promise.all Resolve Element Functions
A Promise.all resolve element function is an anonymous built-in function that is used to
resolve a specific Promise.all element. Each Promise.all resolve element
function has [[Index]], [[Values]], [[Capability]], [[RemainingElements]], and [[AlreadyCalled]] internal slots.
When a Promise.all resolve element function is called with argument x, the
following steps are taken:
b. Return ? Call(promiseCapability.[[Resolve]], undefined, «
valuesArray »).
11. Return undefined.
The "length" property of a Promise.all resolve element function is
1𝔽.
27.2.4.2 Promise.allSettled ( iterable )
This function returns a promise that is fulfilled with an array of promise state snapshots, but only
after all the original promises have settled, i.e. become either fulfilled or rejected. It resolves all
elements of the passed iterable to promises as it runs this algorithm.
g. Let onFulfilled be CreateBuiltinFunction(stepsFulfilled,
lengthFulfilled, "", « [[AlreadyCalled]],
[[Index]], [[Values]], [[Capability]], [[RemainingElements]] »).
h. Let alreadyCalled be the Record
{ [[Value]]: false }.
i. Set onFulfilled.[[AlreadyCalled]] to alreadyCalled.
j. Set onFulfilled.[[Index]] to index.
k. Set onFulfilled.[[Values]] to values.
l. Set onFulfilled.[[Capability]] to resultCapability.
m. Set onFulfilled.[[RemainingElements]] to remainingElementsCount.
p. Let onRejected be CreateBuiltinFunction(stepsRejected,
lengthRejected, "", « [[AlreadyCalled]],
[[Index]], [[Values]], [[Capability]], [[RemainingElements]] »).
q. Set onRejected.[[AlreadyCalled]] to alreadyCalled.
r. Set onRejected.[[Index]] to index.
s. Set onRejected.[[Values]] to values.
t. Set onRejected.[[Capability]] to resultCapability.
u. Set onRejected.[[RemainingElements]] to remainingElementsCount.
v. Set remainingElementsCount.[[Value]] to remainingElementsCount.[[Value]] + 1.
w. Perform ? Invoke(nextPromise, "then",
« onFulfilled, onRejected »).
x. Set index to index + 1.
27.2.4.2.2Promise.allSettled Resolve Element Functions
A Promise.allSettled resolve element function is an anonymous built-in function that is
used to resolve a specific Promise.allSettled element. Each Promise.allSettled
resolve element function has [[Index]], [[Values]],
[[Capability]], [[RemainingElements]], and [[AlreadyCalled]] internal slots.
When a Promise.allSettled resolve element function is called with argument x,
the following steps are taken:
b. Return ? Call(promiseCapability.[[Resolve]], undefined, «
valuesArray »).
15. Return undefined.
The "length" property of a Promise.allSettled resolve element function
is 1𝔽.
27.2.4.2.3Promise.allSettled Reject Element Functions
A Promise.allSettled reject element function is an anonymous built-in function that is
used to reject a specific Promise.allSettled element. Each Promise.allSettled
reject element function has [[Index]], [[Values]],
[[Capability]], [[RemainingElements]], and [[AlreadyCalled]] internal slots.
When a Promise.allSettled reject element function is called with argument x,
the following steps are taken:
b. Return ? Call(promiseCapability.[[Resolve]], undefined, «
valuesArray »).
15. Return undefined.
The "length" property of a Promise.allSettled reject element function
is 1𝔽.
27.2.4.3 Promise.any ( iterable )
This function returns a promise that is fulfilled by the first given promise to be fulfilled, or rejected
with an AggregateError holding the rejection reasons if all of the given promises are
rejected. It resolves all elements of the passed iterable to promises as it runs this algorithm.
g. Let onRejected be CreateBuiltinFunction(stepsRejected,
lengthRejected, "", « [[AlreadyCalled]],
[[Index]], [[Errors]], [[Capability]], [[RemainingElements]] »).
h. Set onRejected.[[AlreadyCalled]] to false.
i. Set onRejected.[[Index]] to index.
j. Set onRejected.[[Errors]] to errors.
k. Set onRejected.[[Capability]] to resultCapability.
l. Set onRejected.[[RemainingElements]] to remainingElementsCount.
m. Set remainingElementsCount.[[Value]] to remainingElementsCount.[[Value]] + 1.
n. Perform ? Invoke(nextPromise, "then",
« resultCapability.[[Resolve]],
onRejected »).
o. Set index to index + 1.
27.2.4.3.2Promise.any Reject Element Functions
A Promise.any reject element function is an anonymous built-in function that is used to
reject a specific Promise.any element. Each Promise.any reject element
function has [[Index]], [[Errors]], [[Capability]], [[RemainingElements]], and [[AlreadyCalled]] internal slots.
When a Promise.any reject element function is called with argument x, the
following steps are taken:
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.
27.2.4.5 Promise.race ( iterable )
This function returns a new promise which is settled in the same way as the first passed promise to
settle. It resolves all elements of the passed iterable to promises as it runs this algorithm.
If the iterable argument yields no values or if none of the promises yielded by
iterable ever settle, then the pending promise returned by this method will never be
settled.
Note 2
This function expects its this value to be a constructor
function that supports the parameter conventions of the Promise constructor. It
also expects that its this value provides a resolve method.
3. Perform ? Call(promiseCapability.[[Reject]], undefined, « r »).
4. Return promiseCapability.[[Promise]].
Note
This function expects its this value to be a constructor
function that supports the parameter conventions of the Promise constructor.
27.2.4.7 Promise.resolve ( x )
This function returns either a new promise resolved with the passed argument, or the argument itself if
the argument is a promise produced by this constructor.
Promise[@@species] is an accessor property whose set
accessor function is undefined. Its get accessor function performs the following steps
when called:
1. Return the this value.
The value of the "name" property of this function is "get
[Symbol.species]".
Note
Promise prototype methods normally use their this value's constructor to create a derived
object. However, a subclass constructor may over-ride that
default behaviour by redefining its @@species property.
a. Let thenFinallyClosure be a new Abstract Closure with
parameters (value) that captures onFinally and C and performs the
following steps when called:
c. Let catchFinallyClosure be a new Abstract Closure with
parameters (reason) that captures onFinally and C and performs
the following steps when called:
The abstract operation PerformPromiseThen takes arguments promise (a Promise),
onFulfilled (an ECMAScript language
value), and onRejected (an ECMAScript
language value) and optional argument resultCapability (a PromiseCapability
Record) and returns an ECMAScript
language value. It performs the “then” operation on promise using
onFulfilled and onRejected as its settlement actions. If
resultCapability is passed, the result is stored by updating resultCapability's
promise. If it is not passed, then PerformPromiseThen is being called by a specification-internal
operation where the result does not matter. It performs the following steps when called:
creates and initializes a new GeneratorFunction when called as a function rather than as a constructor. Thus the function call
GeneratorFunction (…) is equivalent to the object creation expression
new GeneratorFunction (…) with the same arguments.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit
the specified GeneratorFunction behaviour must include a super call to the GeneratorFunction
constructor to create and initialize
subclass instances with the internal slots necessary for built-in GeneratorFunction behaviour. All
ECMAScript syntactic forms for defining generator function objects create direct
instances of GeneratorFunction. There is no syntactic means to create instances of GeneratorFunction
subclasses.
The initial value of the @@toStringTag property is
the String value "GeneratorFunction".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
true }.
27.3.4 GeneratorFunction Instances
Every GeneratorFunction instance is an ECMAScript function object and has the internal
slots listed in Table
30. The value of the [[IsClassConstructor]] internal slot for
all such instances is false.
Each GeneratorFunction instance has the following own properties:
27.3.4.1 length
The specification for the "length" property of Function instances given in 20.2.4.1 also
applies to GeneratorFunction instances.
27.3.4.2 name
The specification for the "name" property of Function instances given in 20.2.4.2 also applies
to GeneratorFunction instances.
27.3.4.3 prototype
Whenever a GeneratorFunction instance is created another ordinary object
is also created and is the initial value of the generator function's "prototype"
property. The value of the prototype property is used to initialize the [[Prototype]] internal slot of a newly created Generator when the generator
function object is invoked using
[[Call]].
This property has the attributes { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]:
false }.
Note
Unlike Function instances, the object that is the value of a GeneratorFunction's
"prototype" property does not have a "constructor" property
whose value is the GeneratorFunction instance.
creates and initializes a new AsyncGeneratorFunction when called as a function rather than as a
constructor. Thus the function call
AsyncGeneratorFunction (...) is equivalent to the object creation expression
new AsyncGeneratorFunction (...) with the same arguments.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit
the specified AsyncGeneratorFunction behaviour must include a super call to the
AsyncGeneratorFunction constructor to create and initialize
subclass instances with the internal slots necessary for built-in AsyncGeneratorFunction behaviour. All
ECMAScript syntactic forms for defining async generator function objects
create direct instances of AsyncGeneratorFunction. There is no syntactic means to create instances of
AsyncGeneratorFunction subclasses.
The initial value of the @@toStringTag property is
the String value "AsyncGeneratorFunction".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
true }.
27.4.4 AsyncGeneratorFunction Instances
Every AsyncGeneratorFunction instance is an ECMAScript function object and has the internal
slots listed in Table
30. The value of the [[IsClassConstructor]] internal slot for
all such instances is false.
Each AsyncGeneratorFunction instance has the following own properties:
27.4.4.1 length
The value of the "length" property is an integral Number
that indicates the typical number of arguments expected by the AsyncGeneratorFunction. However, the
language permits the function to be invoked with some other number of arguments. The behaviour of an
AsyncGeneratorFunction when invoked on a number of arguments other than the number specified by its
"length" property depends on the function.
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
true }.
27.4.4.2 name
The specification for the "name" property of Function instances given in 20.2.4.2 also applies
to AsyncGeneratorFunction instances.
27.4.4.3 prototype
Whenever an AsyncGeneratorFunction instance is created, another ordinary object
is also created and is the initial value of the async generator function's "prototype"
property. The value of the prototype property is used to initialize the [[Prototype]] internal slot of a newly created AsyncGenerator when the generator
function object is invoked using
[[Call]].
This property has the attributes { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]:
false }.
Note
Unlike function instances, the object that is the value of an AsyncGeneratorFunction's
"prototype" property does not have a "constructor" property
whose value is the AsyncGeneratorFunction instance.
27.5 Generator Objects
A Generator is an instance of a generator function and conforms to both the Iterator and
Iterable interfaces.
Generator instances directly inherit properties from the object that is the initial value of the
"prototype" property of the Generator function that created the instance. Generator
instances indirectly inherit properties from the Generator Prototype intrinsic, %GeneratorFunction.prototype.prototype%.
27.5.1 Properties of the Generator Prototype Object
The abstract operation GeneratorStart takes arguments generator (a Generator) and
generatorBody (a FunctionBodyParse Node or an Abstract Closure with no
parameters) and returns unused. It performs the following steps when called:
1. Assert: The value of
generator.[[GeneratorState]] is undefined.
g. Set acGenerator.[[GeneratorState]] to completed.
h. NOTE: Once a generator enters the
completed state it never leaves it and its associated execution context
is never resumed. Any execution state associated with acGenerator can be discarded at
this point.
5. Set the code evaluation state of genContext such that
when evaluation is resumed for that execution context,
closure will be called with no arguments.
6. Set generator.[[GeneratorContext]] to genContext.
7. Set generator.[[GeneratorState]] to suspended-start.
The abstract operation GeneratorValidate takes arguments generator (an ECMAScript language
value) and generatorBrand (a String or empty) and
returns either a normal completion
containing one of suspended-start,
suspended-yield, or completed, or a throw
completion. It performs the following steps when called:
9. Resume the suspended evaluation of
genContext using NormalCompletion(value)
as the result of the operation that suspended it. Let result be the value returned by the
resumed computation.
b. NOTE: Once a generator enters the
completed state it never leaves it and its associated execution context
is never resumed. Any execution state associated with generator can be discarded at
this point.
10. Resume the suspended evaluation of
genContext using abruptCompletion as the result of the operation that
suspended it. Let result be the Completion
Record returned by the resumed computation.
8. Resume callerContext passing NormalCompletion(iterNextObj).
If genContext is ever resumed again, let resumptionValue be the Completion
Record with which it is resumed.
The abstract operation CreateIteratorFromClosure takes arguments closure (an Abstract Closure with no
parameters), generatorBrand (a String or empty), and
generatorPrototype (an Object) and returns a Generator. It performs the following steps when
called:
1. NOTE: closure can contain uses of the Yield
operation to yield an IteratorResult object.
2. Let internalSlotsList be « [[GeneratorState]], [[GeneratorContext]], [[GeneratorBrand]] ».
3. Let generator be OrdinaryObjectCreate(generatorPrototype,
internalSlotsList).
4. Set generator.[[GeneratorBrand]] to generatorBrand.
An AsyncGenerator is an instance of an async generator function and conforms to both the AsyncIterator and
AsyncIterable interfaces.
AsyncGenerator instances directly inherit properties from the object that is the initial value of the
"prototype" property of the AsyncGenerator function that created the instance.
AsyncGenerator instances indirectly inherit properties from the AsyncGenerator Prototype intrinsic, %AsyncGeneratorFunction.prototype.prototype%.
27.6.1 Properties of the AsyncGenerator Prototype Object
Records
which represent requests to resume the async generator. Except during state transitions, it is
non-empty if and only if [[AsyncGeneratorState]] is either
executing or awaiting-return.
[[GeneratorBrand]]
a String or empty
A brand used to distinguish different kinds of async generators. The [[GeneratorBrand]] of async generators declared by ECMAScript source text is
always empty.
27.6.3 AsyncGenerator Abstract Operations
27.6.3.1 AsyncGeneratorRequest Records
An AsyncGeneratorRequest is a Record
value used to store information about how an async generator should be resumed and contains capabilities
for fulfilling or rejecting the corresponding promise.
The abstract operation AsyncGeneratorStart takes arguments generator (an AsyncGenerator) and
generatorBody (a FunctionBodyParse Node or an Abstract Closure with no
parameters) and returns unused. It performs the following steps when called:
1. Assert: generator.[[AsyncGeneratorState]] is undefined.
5. Set the code evaluation state of genContext such that
when evaluation is resumed for that execution context,
closure will be called with no arguments.
6. Set generator.[[AsyncGeneratorContext]] to genContext.
7. Set generator.[[AsyncGeneratorState]] to suspended-start.
8. Set generator.[[AsyncGeneratorQueue]] to a new empty List.
The abstract operation AsyncGeneratorEnqueue takes arguments generator (an AsyncGenerator),
completion (a Completion
Record), and promiseCapability (a PromiseCapability
Record) and returns unused. It performs the following steps when
called:
1. Let request be AsyncGeneratorRequest
{ [[Completion]]: completion, [[Capability]]: promiseCapability }.
2. Append request to generator.[[AsyncGeneratorQueue]].
The abstract operation AsyncGeneratorCompleteStep takes arguments generator (an
AsyncGenerator), completion (a Completion
Record), and done (a Boolean) and optional argument realm (a
Realm
Record) and returns unused. It performs the following steps when
called:
1. Assert: generator.[[AsyncGeneratorQueue]] is not empty.
2. Let next be the first element of
generator.[[AsyncGeneratorQueue]].
3. Remove the first element from generator.[[AsyncGeneratorQueue]].
The abstract operation AsyncGeneratorResume takes arguments generator (an AsyncGenerator) and
completion (a Completion
Record) and returns unused. It performs the following steps when
called:
1. Assert: generator.[[AsyncGeneratorState]] is either suspended-start or
suspended-yield.
2. Let genContext be generator.[[AsyncGeneratorContext]].
7. Resume the suspended evaluation of
genContext using completion as the result of the operation that
suspended it. Let result be the Completion
Record returned by the resumed computation.
The abstract operation AsyncGeneratorAwaitReturn takes argument generator (an AsyncGenerator)
and returns either a normal completion
containingunused or a throw
completion. It performs the following steps when called:
1. Let queue be generator.[[AsyncGeneratorQueue]].
The abstract operation AsyncGeneratorDrainQueue takes argument generator (an AsyncGenerator)
and returns unused. It drains the generator's AsyncGeneratorQueue until it
encounters an AsyncGeneratorRequest
which holds a return
completion. It performs the following steps when called:
1. Assert: generator.[[AsyncGeneratorState]] is completed.
2. Let queue be generator.[[AsyncGeneratorQueue]].
3. If queue is empty, return
unused.
4. Let done be false.
5. Repeat, while done is false,
a. Let next be the first element of queue.
b. Let completion be Completion(next.[[Completion]]).
The abstract operation CreateAsyncIteratorFromClosure takes arguments closure (an Abstract Closure with no
parameters), generatorBrand (a String or empty), and
generatorPrototype (an Object) and returns an AsyncGenerator. It performs the following steps
when called:
1. NOTE: closure can contain uses of the Await
operation and uses of the Yield operation to yield an
IteratorResult object.
2. Let internalSlotsList be « [[AsyncGeneratorState]], [[AsyncGeneratorContext]],
[[AsyncGeneratorQueue]], [[GeneratorBrand]] ».
3. Let generator be OrdinaryObjectCreate(generatorPrototype,
internalSlotsList).
4. Set generator.[[GeneratorBrand]] to generatorBrand.
5. Set generator.[[AsyncGeneratorState]] to undefined.
creates and initializes a new AsyncFunction when called as a function rather than as a constructor. Thus the function call
AsyncFunction(…) is equivalent to the object creation expression
new AsyncFunction(…) with the same arguments.
may be used as the value of an extends clause of a class definition. Subclass constructors that intend to inherit
the specified AsyncFunction behaviour must include a super call to the AsyncFunction
constructor to create and initialize a
subclass instance with the internal slots necessary for built-in async function behaviour. All ECMAScript
syntactic forms for defining async function objects create direct
instances of AsyncFunction. There is no syntactic means to create instances of AsyncFunction subclasses.
The initial value of the @@toStringTag property is
the String value "AsyncFunction".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
true }.
27.7.4 AsyncFunction Instances
Every AsyncFunction instance is an ECMAScript function object and has the internal
slots listed in Table
30. The value of the [[IsClassConstructor]] internal slot for
all such instances is false. AsyncFunction instances are not constructors and do not have a [[Construct]] internal method. AsyncFunction instances do not have a prototype
property as they are not constructible.
Each AsyncFunction instance has the following own properties:
27.7.4.1 length
The specification for the "length" property of Function instances given in 20.2.4.1 also
applies to AsyncFunction instances.
27.7.4.2 name
The specification for the "name" property of Function instances given in 20.2.4.2 also applies
to AsyncFunction instances.
3. NOTE: Copying the execution state is required for AsyncBlockStart to resume
its execution. It is ill-defined to resume a currently executing context.
The abstract operation AsyncBlockStart takes arguments promiseCapability (a PromiseCapability
Record), asyncBody (a Parse
Node), and asyncContext (an execution
context) and returns unused. It performs the following steps
when called:
3. Let closure be a new Abstract Closure with no
parameters that captures promiseCapability and asyncBody and performs the
following steps when called:
ii. Perform ! Call(promiseCapability.[[Reject]], undefined, « result.[[Value]] »).
h. Return
unused.
4. Set the code evaluation state of asyncContext such
that when evaluation is resumed for that execution context,
closure will be called with no arguments.
8. Assert: result is a
normal
completion with a value of unused. The possible sources of
this value are Await or, if the async function doesn't
await anything, step 3.h
above.
A Module Namespace Object is a module namespace exotic
object that provides runtime property-based access to a module's exported bindings. There
is no constructor function for Module Namespace
Objects. Instead, such an object is created for each module that is imported by an ImportDeclaration that contains a
NameSpaceImport.
In addition to the properties specified in 10.4.6
each Module
Namespace Object has the following own property:
28.3.1 @@toStringTag
The initial value of the @@toStringTag property is the
String value "Module".
This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]:
false }.
29 Memory Model
The memory consistency model, or memory model, specifies the possible orderings of
Shared Data Block events,
arising via accessing TypedArray instances backed by a
SharedArrayBuffer and via methods on the Atomics object. When the program has no data races (defined below), the
ordering of events appears as sequentially consistent, i.e., as an interleaving of actions from each agent. When the
program has data races, shared memory operations may appear sequentially inconsistent. For example, programs may
exhibit causality-violating behaviour and other astonishments. These astonishments arise from compiler
transforms and the design of CPUs (e.g., out-of-order execution and speculation). The memory model defines both
the precise conditions under which a program exhibits sequentially consistent behaviour as well as the possible
values read from data races. To wit, there is no undefined behaviour.
The memory model is defined as relational constraints on events introduced by abstract
operations on SharedArrayBuffer or by methods on the Atomics object during an evaluation.
Note
This section provides an axiomatic model on events introduced by the abstract
operations on SharedArrayBuffers. It bears stressing that the model is not expressible
algorithmically, unlike the rest of this specification. The nondeterministic introduction of events by
abstract
operations is the interface between the operational semantics of ECMAScript evaluation
and the axiomatic semantics of the memory model. The semantics of these events is defined by considering
graphs of all events in an evaluation. These are neither Static Semantics nor Runtime Semantics. There is no
demonstrated algorithmic implementation, but instead a set of constraints that determine if a particular
event graph is allowed or disallowed.
29.1 Memory Model Fundamentals
Shared memory accesses (reads and writes) are divided into two groups, atomic accesses and data accesses,
defined below. Atomic accesses are sequentially consistent, i.e., there is a strict total ordering of events
agreed upon by all agents in an agent
cluster. Non-atomic accesses do not have a strict total ordering agreed upon by all
agents,
i.e., unordered.
Note 1
No orderings weaker than sequentially consistent and stronger than unordered, such as release-acquire,
are supported.
A Shared Data Block event is either a ReadSharedMemory, WriteSharedMemory, or ReadModifyWriteSharedMemory Record.
These events are introduced by abstract
operations or by methods on the Atomics object.
Some operations may also introduce Synchronize events. A Synchronize event has no fields, and exists purely to
directly constrain the permitted orderings of other events.
In addition to Shared Data Block and Synchronize
events, there are host-specific events.
Let the range of a ReadSharedMemory, WriteSharedMemory, or ReadModifyWriteSharedMemory event be the Set of
contiguous integers from its [[ByteIndex]] to [[ByteIndex]] + [[ElementSize]] - 1. Two events' ranges are equal when the events have the same [[Block]], and the ranges are element-wise equal. Two events' ranges are overlapping
when the events have the same [[Block]], the ranges are not equal and their
intersection is non-empty. Two events' ranges are disjoint when the events do not have the same [[Block]] or their ranges are neither equal nor overlapping.
Note 2
Examples of host-specific synchronizing events that
should be accounted for are: sending a SharedArrayBuffer from one agent to another (e.g., by
postMessage in a browser), starting and stopping agents, and communicating within the
agent cluster via channels
other than shared memory. It is assumed those events are appended to agent-order
during evaluation like the other SharedArrayBuffer events.
An empty candidate execution is a candidate
execution Record whose
fields are empty Lists and
Relations.
29.5 Abstract Operations for the Memory Model
29.5.1 EventSet ( execution )
The abstract operation EventSet takes argument execution (a candidate execution) and
returns a Set of events. It performs the following steps when called:
The abstract operation SharedDataBlockEventSet takes argument execution (a candidate execution) and
returns a Set of events. It performs the following steps when called:
The abstract operation HostEventSet takes argument execution (a candidate execution) and
returns a Set of events. It performs the following steps when called:
iii. Let bytesModified be W.[[ModifyOp]](bytes, W.[[Payload]]).
iv. Let byte be
bytesModified[payloadIndex].
e. Append byte to bytesRead.
f. Set byteLocation to byteLocation + 1.
4. Return bytesRead.
Note 1
The read-modify-write modification [[ModifyOp]] is given by the function
properties on the Atomics object that introduce ReadModifyWriteSharedMemory
events.
For a candidate executionexecution, execution.[[AgentOrder]] is a Relation on
events that satisfies the following.
For each pair (E, D) in EventSet(execution),
execution.[[AgentOrder]] contains (E, D) if
there is some Agent Events Recordaer in execution.[[EventsRecords]] such that E
and D are in aer.[[EventList]] and E is before
D in List order
of aer.[[EventList]].
If execution.[[HostSynchronizesWith]] contains (E,
D), E and D are in HostEventSet(execution).
There is no cycle in the union of execution.[[HostSynchronizesWith]]
and execution.[[AgentOrder]].
Note 1
For two host-specific events E and
D, E host-synchronizes-with D implies Ehappens-beforeD.
Note 2
The host-synchronizes-with relation allows the host to provide additional synchronization
mechanisms, such as postMessage between HTML workers.
29.6.5 synchronizes-with
For a candidate executionexecution, execution.[[SynchronizesWith]] is the least
Relation on
events that satisfies the following.
For each pair (R, W) in execution.[[ReadsFrom]],
execution.[[SynchronizesWith]] contains (W, R)
if R.[[Order]] is seq-cst, W.[[Order]] is seq-cst, and R and W have
equal ranges.
For each element eventsRecord of execution.[[EventsRecords]], the following is true.
For each pair (S, Sw) in eventsRecord.[[AgentSynchronizesWith]], execution.[[SynchronizesWith]] contains (S, Sw).
For each pair (E, D) in execution.[[HostSynchronizesWith]], execution.[[SynchronizesWith]] contains (E, D).
Note 1
Owing to convention, write events synchronizes-with read events, instead of read events
synchronizes-with write events.
Note 2
init events do not participate in synchronizes-with, and are instead constrained
directly by happens-before.
Note 3
Not all seq-cst events related by reads-from are related by
synchronizes-with. Only events that also have equal ranges are related by synchronizes-with.
For a candidate executionexecution, execution.[[HappensBefore]] is the least Relation on
events that satisfies the following.
For each pair (E, D) in execution.[[AgentOrder]], execution.[[HappensBefore]]
contains (E, D).
For each pair (E, D) in execution.[[SynchronizesWith]], execution.[[HappensBefore]] contains (E, D).
For each pair (E, D) in SharedDataBlockEventSet(execution),
execution.[[HappensBefore]] contains (E, D) if
E.[[Order]] is init and E and
D have overlapping ranges.
For each pair (E, D) in EventSet(execution),
execution.[[HappensBefore]] contains (E, D) if
there is an event F such that the pairs (E, F) and (F,
D) are in execution.[[HappensBefore]].
Note
Because happens-before is a superset of agent-order, candidate executions
are consistent with the single-thread evaluation semantics of ECMAScript.
29.7 Properties of Valid Executions
29.7.1 Valid Chosen Reads
A candidate executionexecution has valid chosen reads if the following algorithm returns true.
i. If execution.[[HappensBefore]] contains (R, W), then
1. Return false.
ii. If there exists a WriteSharedMemory
or ReadModifyWriteSharedMemory
event V that has byteLocation in its range such that the pairs
(W, V) and (V, R) are in execution.[[HappensBefore]], then
1. Return false.
iii. Set byteLocation to byteLocation +
1.
2. Return true.
29.7.3 Tear Free Reads
A candidate executionexecution has tear free reads if the following algorithm returns true.
i. Assert: The remainder of dividing
R.[[ByteIndex]] by R.[[ElementSize]] is 0.
ii. For each event W such that
execution.[[ReadsFrom]] contains (R,
W) and W.[[NoTear]] is true,
do
1. If R and W have equal ranges and
there exists an event V such that V and W have equal
ranges, V.[[NoTear]] is true,
W is not V, and execution.[[ReadsFrom]] contains (R, V), then
a. Return false.
2. Return true.
Note
An event's [[NoTear]] field is true when that event was
introduced via accessing an integerTypedArray, and
false when introduced via accessing a floating point TypedArray or
DataView.
Intuitively, this requirement says when a memory range is accessed in an aligned fashion via an
integerTypedArray, a single
write event on that range must "win" when in a data race with other write events with equal ranges. More
precisely, this requirement says an aligned read event cannot read a value composed of bytes from
multiple, different write events all with equal ranges. It is possible, however, for an aligned read
event to read from multiple write events with overlapping ranges.
For each pair (E, D) in execution.[[HappensBefore]], (E, D) is in memory-order.
For each pair (R, W) in execution.[[ReadsFrom]], there is no WriteSharedMemory
or ReadModifyWriteSharedMemory
event V in SharedDataBlockEventSet(execution)
such that V.[[Order]] is seq-cst, the pairs
(W, V) and (V, R) are in memory-order, and any of the
following conditions are true.
execution.[[SynchronizesWith]] contains the pair (W,
R), and V and R have equal ranges.
The pairs (W, R) and (V, R) are in
execution.[[HappensBefore]], W.[[Order]] is seq-cst, and W and V
have equal ranges.
The pairs (W, R) and (W, V) are in
execution.[[HappensBefore]], R.[[Order]] is seq-cst, and V and R
have equal ranges.
Note 1
This clause additionally constrains seq-cst events on equal ranges.
This clause together with the forward progress guarantee on agents ensure the
liveness condition that seq-cst writes become visible to
seq-cst reads with equal range in finite time.
A candidate execution has
sequentially consistent atomics if a memory-order exists.
Note 3
While memory-order includes all events in EventSet(execution),
those that are not constrained by happens-before or synchronizes-with are
allowed to occur anywhere in the order.
29.7.5 Valid Executions
A candidate executionexecution is a valid execution (or simply an execution) if all of the following are true.
ii. If execution.[[ReadsFrom]] contains either (E, D) or (D,
E), then
1. Return true.
2. Return false.
29.9 Data Races
For an execution execution, two events E and D in SharedDataBlockEventSet(execution)
are in a data race if the following algorithm returns true.
1. If E and D are in a race in
execution, then
a. If E.[[Order]] is not
seq-cst or D.[[Order]] is not
seq-cst, then
i. Return true.
b. If E and D have overlapping ranges, then
i. Return true.
2. Return false.
29.10 Data Race Freedom
An execution execution is data race free if there are no two events in SharedDataBlockEventSet(execution)
that are in a data race.
A program is data race free if all its executions are data race free.
The memory model guarantees sequential
consistency of all events for data race free programs.
29.11 Shared Memory Guidelines
Note 1
The following are guidelines for ECMAScript programmers working with shared memory.
We recommend programs be kept data race free, i.e., make it so that it is impossible for there to be
concurrent non-atomic operations on the same memory location. Data race free programs have interleaving
semantics where each step in the evaluation semantics of each agent are interleaved with each other. For
data race free programs, it is not necessary to understand the details of the memory
model. The details are unlikely to build intuition that will help one to better write
ECMAScript.
More generally, even if a program is not data race free it may have predictable behaviour, so long as
atomic operations are not involved in any data races and the operations that race all have the same access
size. The simplest way to arrange for atomics not to be involved in races is to ensure that different
memory cells are used by atomic and non-atomic operations and that atomic accesses of different sizes are
not used to access the same cells at the same time. Effectively, the program should treat shared memory as
strongly typed as much as possible. One still cannot depend on the ordering and timing of non-atomic
accesses that race, but if memory is treated as strongly typed the racing accesses will not "tear" (bits
of their values will not be mixed).
Note 2
The following are guidelines for ECMAScript implementers writing compiler transformations for programs
using shared memory.
It is desirable to allow most program transformations that are valid in a single-agent setting in a
multi-agent setting, to ensure that the
performance of each agent in a multi-agent program is as good as
it would be in a single-agent setting. Frequently these
transformations are hard to judge. We outline some rules about program transformations that are intended
to be taken as normative (in that they are implied by the memory model or
stronger than what the memory model implies) but which
are likely not exhaustive. These rules are intended to apply to program transformations that precede the
introductions of the events that make up the agent-order.
Let an agent-order slice be the subset of the
agent-order pertaining to a single
agent.
Let possible read values of a read event be the set of all values of ValueOfReadEvent for that
event across all valid executions.
Any transformation of an agent-order slice that is valid in the absence of shared memory is valid in the
presence of shared memory, with the following exceptions.
Atomics are carved in stone: Program transformations must not cause the
seq-cst events in an agent-order slice to be reordered with its
unordered operations, nor its seq-cst operations to be
reordered with each other, nor may a program transformation remove a seq-cst
operation from the agent-order.
(In practice, the prohibition on reorderings forces a compiler to assume that every
seq-cst operation is a synchronization and included in the final memory-order, which it would
usually have to assume anyway in the absence of inter-agent program analysis. It also forces
the compiler to assume that every call where the callee's effects on the memory-order
are unknown may contain seq-cst operations.)
Reads must be stable: Any given shared memory read must only observe a single value in an
execution.
(For example, if what is semantically a single read in the program is executed multiple times then
the program is subsequently allowed to observe only one of the values read. A transformation known as
rematerialization can violate this rule.)
Writes must be stable: All observable writes to shared memory must follow from program
semantics in an execution.
(For example, a transformation may not introduce certain observable writes, such as by using
read-modify-write operations on a larger location to write a smaller datum, writing a value to memory
that the program could not have written, or writing a just-read value back to the location it was read
from, if that location could have been overwritten by another agent after the read.)
Possible read values must be non-empty: Program transformations cannot cause the possible
read values of a shared memory read to become empty.
(Counterintuitively, this rule in effect restricts transformations on writes, because writes have
force in memory model insofar as to be
read by read events. For example, writes may be moved and coalesced and sometimes reordered between
two seq-cst operations, but the transformation may not remove every write that
updates a location; some write must be preserved.)
Examples of transformations that remain valid are: merging multiple non-atomic reads from the same
location, reordering non-atomic reads, introducing speculative non-atomic reads, merging multiple
non-atomic writes to the same location, reordering non-atomic writes to different locations, and hoisting
non-atomic reads out of loops even if that affects termination. Note in general that aliased TypedArrays make it hard to prove that
locations are different.
Note 3
The following are guidelines for ECMAScript implementers generating machine code for shared memory
accesses.
For architectures with memory models no weaker than those of ARM or Power, non-atomic stores and loads
may be compiled to bare stores and loads on the target architecture. Atomic stores and loads may be
compiled down to instructions that guarantee sequential consistency. If no such instructions exist, memory
barriers are to be employed, such as placing barriers on both sides of a bare store or load.
Read-modify-write operations may be compiled to read-modify-write instructions on the target architecture,
such as LOCK-prefixed instructions on x86, load-exclusive/store-exclusive instructions on
ARM, and load-link/store-conditional instructions on Power.
Specifically, the memory model is intended to allow
code generation as follows.
Every atomic operation in the program is assumed to be necessary.
Atomic operations are never rearranged with each other or with non-atomic operations.
Functions are always assumed to perform atomic operations.
Atomic operations are never implemented as read-modify-write operations on larger data, but as
non-lock-free atomics if the platform does not have atomic operations of the appropriate size. (We
already assume that every platform has normal memory access operations of every interesting size.)
Naive code generation uses these patterns:
Regular loads and stores compile to single load and store instructions.
Lock-free atomic loads and stores compile to a full (sequentially consistent) fence, a regular load or
store, and a full fence.
Lock-free atomic read-modify-write accesses compile to a full fence, an atomic read-modify-write
instruction sequence, and a full fence.
Non-lock-free atomics compile to a spinlock acquire, a full fence, a series of non-atomic load and
store instructions, a full fence, and a spinlock release.
That mapping is correct so long as an atomic operation on an address range does not race with a
non-atomic write or with an atomic operation of different size. However, that is all we need: the
memory model effectively demotes
the atomic operations involved in a race to non-atomic status. On the other hand, the naive mapping is
quite strong: it allows atomic operations to be used as sequentially consistent fences, which the
memory model does not actually
guarantee.
Local improvements to those basic patterns are also allowed, subject to the constraints of the memory
model. For example:
There are obvious platform-dependent improvements that remove redundant fences. For example, on x86
the fences around lock-free atomic loads and stores can always be omitted except for the fence following
a store, and no fence is needed for lock-free read-modify-write instructions, as these all use
LOCK-prefixed instructions. On many platforms there are fences of several strengths, and
weaker fences can be used in certain contexts without destroying sequential consistency.
Most modern platforms support lock-free atomics for all the data sizes required by ECMAScript atomics.
Should non-lock-free atomics be needed, the fences surrounding the body of the atomic operation can
usually be folded into the lock and unlock steps. The simplest solution for non-lock-free atomics is to
have a single lock word per SharedArrayBuffer.
There are also more complicated platform-dependent local improvements, requiring some code analysis.
For example, two back-to-back fences often have the same effect as a single fence, so if code is
generated for two atomic operations in sequence, only a single fence need separate them. On x86, even a
single fence separating atomic stores can be omitted, as the fence following a store is only needed to
separate the store from a subsequent load.
The ECMAScript language syntax and semantics defined in this annex are required when the ECMAScript host is a web
browser. The content of this annex is normative but optional if the ECMAScript host is not a web browser.
Note
This annex describes various legacy features and other characteristics of web browser ECMAScript hosts. All of the
language features and behaviours specified in this annex have one or more undesirable characteristics and in
the absence of legacy usage would be removed from this specification. However, the usage of these features
by large numbers of existing web pages means that web browsers must continue to support them. The
specifications in this annex define the requirements for interoperable implementations of these legacy
features.
These features are not considered part of the core ECMAScript language. Programmers should not use or
assume the existence of these features and behaviours when writing new ECMAScript code. ECMAScript
implementations are discouraged from implementing these features unless the implementation is part of a web
browser or is required to run the same legacy ECMAScript code that web browsers encounter.
B.1 Additional Syntax
B.1.1 HTML-like Comments
The syntax and semantics of 12.4 is extended as follows except that
this extension is not allowed when parsing source text using the goal
symbolModule:
The syntax of 22.2.1 is modified and extended as
follows. These changes introduce ambiguities that are broken by the ordering of grammar productions and by
contextual information. When parsing using the following grammar, each alternative is considered only if
previous production alternatives do not match.
This alternative pattern grammar and semantics only changes the syntax and semantics of BMP patterns. The
following grammar extensions include productions parameterized with the [UnicodeMode] parameter. However,
none of these extensions change the syntax of Unicode patterns recognized when parsing with the
[UnicodeMode] parameter present on the goal symbol.
1. Return the CharSet containing the single
character \ U+005C (REVERSE SOLIDUS).
Note
This production can only be reached from the sequence \c within a
character class where it is not followed by an acceptable control character.
B.1.2.8.1 CharacterRangeOrUnion ( rer, A,
B )
The abstract operation CharacterRangeOrUnion takes arguments rer (a RegExp Record), A
(a CharSet), and B (a
CharSet) and returns a CharSet. It performs the
following steps when called:
The abstract operation ParsePattern takes arguments
patternText (a sequence of Unicode code points), u (a Boolean), and v (a
Boolean). It performs the following steps when called:
1. If v is true and u is
true, then
a. Let parseResult be a List
containing one or more SyntaxError objects.
2. Else if v is true, then
a. Let parseResult be ParseText(patternText,
Pattern[+UnicodeMode,
+UnicodeSetsMode, +NamedCaptureGroups]).
3. Else if u is true, then
a. Let parseResult be ParseText(patternText,
Pattern[+UnicodeMode,
~UnicodeSetsMode, +NamedCaptureGroups]).
4. Else,
a. Let parseResult be ParseText(patternText,
Pattern[~UnicodeMode,
~UnicodeSetsMode, ~NamedCaptureGroups]).
b. If parseResult is a Parse Node and
parseResult contains a GroupName, then
i. Set parseResult to ParseText(patternText,
Pattern[~UnicodeMode,
~UnicodeSetsMode, +NamedCaptureGroups]).
5. Return parseResult.
B.2 Additional Built-in Properties
When the ECMAScript host is a web browser the following additional
properties of the standard built-in objects are defined.
This function is a property of the global object. It computes a new
version of a String value in which certain code units have been replaced by a hexadecimal escape sequence.
When replacing a code unit of numeric value less than or equal to 0x00FF, a two-digit escape sequence of
the form %xx is used. When replacing a code unit of numeric value strictly greater
than 0x00FF, a four-digit escape sequence of the form %uxxxx is used.
The encoding is partly based on the encoding described in RFC 1738, but the entire encoding specified
in this standard is described above without regard to the contents of RFC 1738. This encoding does not
reflect changes to RFC 1738 made by RFC 3986.
B.2.1.2 unescape ( string )
This function is a property of the global object. It computes a new
version of a String value in which each escape sequence of the sort that might be introduced by the
escape function is replaced with the code unit that it represents.
B.2.2 Additional Properties of the String.prototype Object
B.2.2.1 String.prototype.substr ( start, length )
This method returns a substring of the result of converting the
this value to a String, starting from index start and running for
length code units (or through the end of the String if length is
undefined). If start is negative, it is treated as sourceLength + start where sourceLength is the
length of the String. The result is a
String value, not a String object.
8. If length is undefined, let
intLength be size; otherwise let intLength be ? ToIntegerOrInfinity(length).
9. Set intLength to the result of clampingintLength between
0 and size.
10. Let intEnd be min(intStart +
intLength, size).
11. Return the substring of S from
intStart to intEnd.
Note
This method is intentionally generic; it does not require that its this value be a
String object. Therefore it can be transferred to other kinds of objects for use as a method.
B.2.2.2 String.prototype.anchor ( name )
This method performs the following steps when called:
b. Let escapedV be the String value that is the
same as V except that each occurrence of the code unit 0x0022 (QUOTATION MARK) in
V has been replaced with the six code unit sequence """.
The property "trimStart" is preferred. The "trimLeft" property
is provided principally for compatibility with old code. It is recommended that the
"trimStart" property be used in new ECMAScript code.
The initial value of the "trimLeft" property is %String.prototype.trimStart%, defined
in 22.1.3.34.
B.2.2.16 String.prototype.trimRight ( )
Note
The property "trimEnd" is preferred. The "trimRight" property
is provided principally for compatibility with old code. It is recommended that the
"trimEnd" property be used in new ECMAScript code.
The initial value of the "trimRight" property is %String.prototype.trimEnd%, defined
in 22.1.3.33.
B.2.3 Additional Properties of the Date.prototype Object
B.2.3.1 Date.prototype.getYear ( )
Note
The getFullYear method is preferred for nearly all purposes, because it avoids the “year
2000 problem.”
This method performs the following steps when called:
This method completely reinitializes the this value RegExp with a new pattern and
flags. An implementation may interpret use of this method as an assertion that the resulting RegExp
object will be used multiple times and hence is a candidate for extra optimization.
B.3 Other Additional Features
B.3.1 Labelled Function Declarations
Prior to ECMAScript 2015, the specification of LabelledStatement did not allow
for the association of a statement label with a FunctionDeclaration. However,
a labelled FunctionDeclaration was an
allowable extension for non-strict code and most
browser-hosted ECMAScript implementations supported that extension. In ECMAScript 2015 and later, the
grammar production for LabelledStatement permits use
of FunctionDeclaration as a
LabelledItem
but 14.13.1
includes an Early Error rule that produces a Syntax Error if that occurs. That rule is modified with the
addition of the highlighted text:
B.3.2 Block-Level Function Declarations Web Legacy Compatibility Semantics
Prior to ECMAScript 2015, the ECMAScript specification did not define the occurrence of a FunctionDeclaration as an
element of a Block
statement's StatementList. However, support for
that form of FunctionDeclaration was an
allowable extension and most browser-hosted ECMAScript implementations permitted them. Unfortunately, the
semantics of such declarations differ among those implementations. Because of these semantic differences,
existing web ECMAScript source text that uses
Block level function
declarations is only portable among browser implementations if the usage only depends upon the semantic
intersection of all of the browser implementations for such declarations. The following are the use cases
that fall within that intersection semantics:
A function is declared and only referenced within a single block.
One or more FunctionDeclarations
whose BindingIdentifier is the
name f occur within the function code of an enclosing function g and that
declaration is nested within a Block.
No other declaration of f that is not a var declaration occurs within the
function code of g.
A function is declared and possibly used within a single Block but also referenced by an inner
function definition that is not contained within that same Block.
One or more FunctionDeclarations
whose BindingIdentifier is the
name f occur within the function code of an enclosing function g and that
declaration is nested within a Block.
No other declaration of f that is not a var declaration occurs within the
function code of g.
There is at least one occurrence of f as an IdentifierReference
within another function h that is nested within g and no other declaration of
f shadows the references to f from within h.
All invocations of h occur after the declaration of f has been evaluated.
A function is declared and possibly used within a single block but also referenced within subsequent
blocks.
One or more FunctionDeclaration
whose BindingIdentifier is the
name f occur within the function code of an enclosing function g and that
declaration is nested within a Block.
No other declaration of f that is not a var declaration occurs within the
function code of g.
There is at least one occurrence of f as an IdentifierReference
within the function code of g that lexically follows the Block containing the declaration of
f.
The first use case is interoperable with the semantics of Block level function declarations provided
by ECMAScript 2015. Any pre-existing ECMAScript source text that employs
that use case will operate using the Block level function declarations semantics defined by clauses
10,
14,
and 15.
ECMAScript 2015 interoperability for the second and third use cases requires the following extensions to
the clause 10,
clause 15,
clause 19.2.1 and clause 16.1.7 semantics.
If an ECMAScript implementation has a mechanism for reporting diagnostic warning messages, a warning should
be produced when code contains a FunctionDeclaration for which
these compatibility semantics are applied and introduce observable differences from non-compatibility
semantics. For example, if a var binding is not introduced because its introduction would create an
early
error, a warning message should not be produced.
B.3.2.1 Changes to FunctionDeclarationInstantiation
1. NOTE: A var binding for F is only
instantiated here if it is neither a VarDeclaredName, the name of a formal parameter, or
another FunctionDeclaration.
2. If instantiatedVarNames does not contain
F and F is not "arguments", then
a. Perform
! varEnv.CreateMutableBinding(F, false).
b. Perform
! varEnv.InitializeBinding(F,
undefined).
The Block of a
Catch clause may
contain var declarations that bind a name that is also bound by the CatchParameter.
At runtime,
such bindings are instantiated in the VariableDeclarationEnvironment. They do not shadow the same-named
bindings introduced by the CatchParameter and hence the
Initializer
for such
var declarations will assign to the corresponding catch parameter rather than the
var binding.
This modified behaviour also applies to var and function declarations introduced
by direct
eval calls contained within the Block of a Catch clause. This change is accomplished
by modifying the algorithm of 19.2.1.3 as
follows:
Objects with an [[IsHTMLDDA]] internal slot are never created by this
specification. However, the document.all
object in web browsers is a host-definedexotic
object with this slot that exists for web compatibility purposes. There are no other
known examples of this type of object and implementations should not create any with the exception of
document.all.
Assignment to an undeclared identifier or otherwise unresolvable reference does not create a property in the
global object. When a simple
assignment occurs within strict mode code, its LeftHandSideExpression must
not evaluate to an unresolvable Reference. If it does a ReferenceError exception is thrown
(6.2.5.6). The LeftHandSideExpression also
may not be a reference to a data property with the attribute value
{ [[Writable]]: false }, to an accessor property
with the attribute value { [[Set]]: undefined }, nor to a
non-existent property of an object whose [[Extensible]] internal slot is
false. In these cases a TypeError exception is thrown (13.15).
Arguments objects for strict functions do not dynamically
share their array-indexed property values with the
corresponding formal parameter bindings of their functions. (10.4.4).
For strict functions, if an arguments
object is created the binding of the local identifier arguments to the arguments object is
immutable and hence may not be the target of an assignment expression. (10.2.11).
Strict mode eval code cannot instantiate variables or functions in the variable environment of the caller to
eval. Instead, a new variable environment is created and that environment is used for declaration binding
instantiation for the eval code (19.2.1).
If this is evaluated within strict mode code, then the
this value is not coerced to an object. A this value of either
undefined or null is not converted to the global
object and primitive values are not converted to wrapper objects. The
this value passed via a function call (including calls made using
Function.prototype.apply and Function.prototype.call) do not coerce the passed
this value to an object (10.2.1.2, 20.2.3.1, 20.2.3.3).
When a delete operator occurs within strict mode code, a
SyntaxError is thrown if its UnaryExpression is a direct
reference to a variable, function argument, or function name (13.5.1.1).
When a delete operator occurs within strict mode code, a
TypeError is thrown if the property to be deleted has the attribute { [[Configurable]]: false } or otherwise cannot be deleted (13.5.1.2).
An implementation may not extend, beyond that defined in this specification, the meanings within strict
functions of properties named "caller" or "arguments"
of function instances.
Preparation steps before, and cleanup steps after, invocation of JobAbstract
Closures. See 9.5.
D.5 Internal Methods of Exotic Objects
Any of the essential internal methods in Table 4 for any
exotic
object not specified within this specification.
D.6 Built-in Objects and Methods
Any built-in objects and methods not defined within this specification, except as restricted in 17.1.
E Corrections and Clarifications in ECMAScript 2015 with Possible Compatibility
Impact
9.1.1.4.15-9.1.1.4.18 Edition 5 and
5.1 used a property existence test to determine whether a global object property corresponding
to a new global declaration already existed. ECMAScript 2015 uses an own property existence test. This
corresponds to what has been most commonly implemented by web browsers.
10.4.2.1:
The 5th Edition moved the capture of the current array length prior to the integer conversion of the
array
index or new length value. However, the captured length value could become invalid if the
conversion process has the side-effect of changing the array length. ECMAScript 2015 specifies that the current
array length must be captured after the possible occurrence of such side-effects.
21.4.1.31: Previous editions permitted the
TimeClip abstract operation to return
either +0𝔽 or -0𝔽 as the representation of a 0
time value. ECMAScript
2015 specifies that +0𝔽 always returned. This means that for ECMAScript 2015 the
time value of a Date is
never observably -0𝔽 and methods that return time values never return
-0𝔽.
21.4.1.32: If a UTC offset
representation is not present, the local time zone is used. Edition 5.1 incorrectly stated that a missing time
zone should be interpreted as "z".
21.4.4.36: If the year
cannot be represented using the Date Time String Format specified in 21.4.1.32 a RangeError
exception is thrown. Previous editions did not specify the behaviour for that case.
21.4.4.41: Previous editions
did not specify the value returned by Date.prototype.toString when the time value is
NaN. ECMAScript 2015 specifies the result to be the String value "Invalid
Date".
22.2.4.1, 22.2.6.13.1: Any LineTerminator
code points in the value of the "source" property of a RegExp instance must be expressed
using an escape sequence. Edition 5.1 only required the escaping of /.
22.2.6.8, 22.2.6.11: In previous
editions, the specifications for String.prototype.match and String.prototype.replace
was incorrect for cases where the pattern argument was a RegExp value whose global flag is set. The
previous specifications stated that for each attempt to match the pattern, if lastIndex did not
change, it should be incremented by 1. The correct behaviour is that lastIndex should be
incremented by 1 only if the pattern matched the empty String.
23.1.3.30: Previous editions
did not specify how a NaN value returned by a comparefn was interpreted by
Array.prototype.sort. ECMAScript 2015 specifies that such as value is treated as if
+0𝔽 was returned from the comparefn. ECMAScript 2015 also specifies
that ToNumber is applied to the result returned
by a comparefn. In previous editions, the effect of a comparefn result that is not a
Number value was implementation-defined. In
practice, implementations call ToNumber.
F Additions and Changes That Introduce Incompatibilities with Prior Editions
6.2.5: In
ECMAScript 2015, Function calls are not allowed to return a Reference
Record.
9.3: In ECMAScript 2018, Template
objects are canonicalized based on Parse Node (source location),
instead of across all occurrences of that template literal or tagged template in a Realm in previous editions.
12.2: In ECMAScript 2016, Unicode 8.0.0
or higher is mandated, as opposed to ECMAScript 2015 which mandated Unicode 5.1. In particular, this caused
U+180E MONGOLIAN VOWEL SEPARATOR, which was in the Space_Separator (Zs) category and
thus treated as whitespace in ECMAScript 2015, to be moved to the Format (Cf) category
(as of Unicode 6.3.0). This causes whitespace-sensitive methods to behave differently. For example,
"\u180E".trim().length was 0 in previous editions, but 1 in ECMAScript
2016 and later. Additionally, ECMAScript 2017 mandated always using the latest version of the Unicode Standard.
12.7: In ECMAScript 2015, the
valid code points for an IdentifierName are specified in terms
of the Unicode properties “ID_Start” and “ID_Continue”. In previous editions, the valid IdentifierName or
Identifier code
points were specified by enumerating various Unicode code point categories.
12.10.1: In
ECMAScript 2015, Automatic Semicolon Insertion adds a semicolon at the end of a do-while statement if the
semicolon is missing. This change aligns the specification with the actual behaviour of most existing
implementations.
13.2.5.1:
In ECMAScript 2015, it is no longer an early error to have duplicate property names
in Object Initializers.
13.15.1:
In ECMAScript 2015, strict mode code containing an
assignment to an immutable binding such as the function name of a FunctionExpression does not
produce an early
error. Instead it produces a runtime
error.
14.2: In
ECMAScript 2015, a StatementList
beginning with the token let followed by the input elements LineTerminator then Identifier is the start
of a LexicalDeclaration. In previous
editions, automatic semicolon insertion would always insert a semicolon before the Identifier input element.
14.7: In ECMAScript 2015, if
the ( token of a for statement is immediately followed by the token sequence let [
then the let is treated as the start of a LexicalDeclaration. In previous
editions such a token sequence would be the start of an Expression.
14.7: In ECMAScript 2015, if
the ( token of a for-in statement is immediately followed by the token sequence let [ then the
let is treated as the start of a ForDeclaration. In previous editions
such a token sequence would be the start of an LeftHandSideExpression.
14.7: Prior to ECMAScript 2015,
an initialization expression could appear as part of the VariableDeclaration that precedes
the inkeyword. In ECMAScript
2015, the ForBinding in
that same position does not
allow the occurrence of such an initializer. In ECMAScript 2017, such an initializer is permitted only in
non-strict
code.
14.15: In ECMAScript 2015, it is an
early
error for a Catch clause to contain a var
declaration for the same Identifier that appears as the Catch clause parameter. In
previous editions, such a variable declaration would be instantiated in the enclosing variable environment but
the declaration's Initializer value would be assigned to
the Catch parameter.
14.15, 19.2.1.3: In ECMAScript
2015, a runtime SyntaxError is thrown if a Catch clause evaluates a non-strict direct
eval whose eval code includes a var or FunctionDeclaration declaration
that binds the same Identifier that appears as the Catch clause parameter.
15.4.5
In ECMAScript 2015, the function objects that are created as the
values of the [[Get]] or [[Set]] attribute of accessor
properties in an ObjectLiteral are not constructor
functions and they do not have a
"prototype" own property. In the previous edition, they were constructors and had a
"prototype" property.
20.1.2.6: In ECMAScript 2015, if the
argument to Object.freeze is not an object it is treated as if it was a non-extensible ordinary
object with no own properties. In the previous edition, a non-object argument always causes a
TypeError to be thrown.
20.1.2.8: In
ECMAScript 2015, if the argument to Object.getOwnPropertyDescriptor is not an object an attempt is
made to coerce the argument using ToObject. If the coercion is successful the
result is used in place of the original argument value. In the previous edition, a non-object argument always
causes a TypeError to be thrown.
20.1.2.10: In ECMAScript
2015, if the argument to Object.getOwnPropertyNames is not an object an attempt is made to coerce
the argument using ToObject. If the coercion is successful the
result is used in place of the original argument value. In the previous edition, a non-object argument always
causes a TypeError to be thrown.
20.1.2.12: In ECMAScript 2015,
if the argument to Object.getPrototypeOf is not an object an attempt is made to coerce the argument
using ToObject. If the coercion is successful the
result is used in place of the original argument value. In the previous edition, a non-object argument always
causes a TypeError to be thrown.
20.1.2.16: In ECMAScript 2015,
if the argument to Object.isExtensible is not an object it is treated as if it was a non-extensible
ordinary
object with no own properties.
In the previous edition, a non-object argument always causes a TypeError to be thrown.
20.1.2.17: In ECMAScript 2015, if
the argument to Object.isFrozen is not an object it is treated as if it was a non-extensible
ordinary
object with no own properties.
In the previous edition, a non-object argument always causes a TypeError to be thrown.
20.1.2.18: In ECMAScript 2015, if
the argument to Object.isSealed is not an object it is treated as if it was a non-extensible
ordinary
object with no own properties.
In the previous edition, a non-object argument always causes a TypeError to be thrown.
20.1.2.19: In ECMAScript 2015, if the
argument to Object.keys is not an object an attempt is made to coerce the argument using ToObject.
If the coercion is successful the result is used in place of the original argument value. In the previous
edition, a non-object argument always causes a TypeError to be thrown.
20.1.2.20: In ECMAScript
2015, if the argument to Object.preventExtensions is not an object it is treated as if it was a
non-extensible ordinary object with no own properties.
In the previous edition, a non-object argument always causes a TypeError to be thrown.
20.1.2.22: In ECMAScript 2015, if the
argument to Object.seal is not an object it is treated as if it was a non-extensible ordinary
object with no own properties. In the previous edition, a non-object argument always causes a
TypeError to be thrown.
20.2.3.2: In ECMAScript
2015, the [[Prototype]] internal slot of a bound function is set to the [[GetPrototypeOf]] value of its target function. In the previous edition, [[Prototype]] was always set to %Function.prototype%.
20.2.4.1: In ECMAScript
2015, the "length" property of function instances is configurable. In previous editions it
was non-configurable.
21.4.4 In
ECMAScript 2015, the Date prototype
object is not a Date instance. In previous editions it was a Date instance whose TimeValue
was NaN.
22.1.3.12 In
ECMAScript 2015, the String.prototype.localeCompare function must treat Strings that are
canonically equivalent according to the Unicode Standard as being identical. In previous editions
implementations were permitted to ignore canonical equivalence and could instead use a bit-wise comparison.
22.1.3.28 and 22.1.3.30 In ECMAScript
2015, lowercase/upper conversion processing operates on code points. In previous editions such the conversion
processing was only applied to individual code units. The only affected code points are those in the Deseret
block of Unicode.
22.1.3.32 In ECMAScript 2015,
the String.prototype.trim method is defined to recognize white space code points that may exist
outside of the Unicode BMP. However, as of Unicode 7 no such code points are defined. In previous editions such
code points would not have been recognized as white space.
22.2.4.1 In ECMAScript 2015, If
the pattern argument is a RegExp instance and the flags argument is not
undefined, a new RegExp instance is created just like pattern except that
pattern's flags are replaced by the argument flags. In previous editions a
TypeError exception was thrown when pattern was a RegExp instance and
flags was not undefined.
22.2.6 In
ECMAScript 2015, the RegExp prototype
object is not a RegExp instance. In previous editions it was a RegExp instance whose pattern
is the empty String.
25.4.15: In ECMAScript 2019,
Atomics.wake has been renamed to Atomics.notify to prevent confusion with
Atomics.wait.
27.1.4.4,
27.6.3.6: In ECMAScript 2019,
the number of Jobs
enqueued by await was reduced,
which could create an observable difference in resolution order between a then() call and an
await expression.
G Colophon
This specification is authored on GitHub in a plaintext source
format called Ecmarkup. Ecmarkup is an HTML and Markdown
dialect that provides a framework and toolset for authoring ECMAScript specifications in plaintext and
processing the specification into a full-featured HTML rendering that follows the editorial conventions for this
document. Ecmarkup builds on and integrates a number of other formats and technologies including Grammarkdown for defining syntax and Ecmarkdown for authoring algorithm steps. PDF renderings of
this specification are produced by printing the HTML rendering to a PDF.
Prior editions of this specification were authored using Word—the Ecmarkup source text that formed the basis of
this edition was produced by converting the ECMAScript 2015 Word document to Ecmarkup using an automated
conversion tool.
H Bibliography
IEEE 754-2019: IEEE Standard for Floating-Point Arithmetic. Institute of
Electrical and Electronic Engineers, New York (2019)
Note
There are no normative changes between IEEE 754-2008 and IEEE 754-2019 that affect the ECMA-262
specification.
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